Buying Guide on Best Reverse Osmosis Systems for Aquariums

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If you are a novice in owning a fish tank, it is easy to become overwhelmed with the array of choices and research. Therefore, this buying guide aims to condense all the important factors that you need to factor into your decision making.

First, you need to understand and differentiate between the 3 most common aquariums; Freshwater, Saltwater/Marine and Reef aquarium. Following this, it is necessary to understand the process of reverse osmosis and why purified water is beneficial for your aquarium.

Afterwards, we will explain the main components of a reverse osmosis unit as well as the process and set-up of most reverse osmosis tanks. In the last part, we will wrap up this article with our top 6 picks of reverse osmosis units.

After reading this, you should have little problem with owning, setting up and maintaining your first fish tank and reverse osmosis unit.

Types of Fish Tanks

Freshwater Aquariums


A freshwater tropical aquarium is the most ideal type of aquarium for beginners. This type of aquarium is also inexpensive as only basic water conditioners such as chlorine removers are added. You are also not required to use luxurious light fixtures or incredibly complex aquarium equipment. For your freshwater aquarium, you just need proper filtration, good water chemistry and lighting.

In a freshwater tropical aquarium, the water temperature ranges from 23 to 29 Degrees Celsius which provides an ideal environment for freshwater tropical fish. As a beginner, you have a vast array of affordable freshwater fish to choose from. We would advise you to try keeping a freshwater aquarium first, before attempting to own a more complex aquarium such as a reef or marine aquarium.

Selecting Freshwater Fish

Freshwater Fish

If you are a novice, you should consult a salesperson on the type of freshwater, tropical fish that you should get. They should give you tips on which fishes are compatible with one another, and so forth. You should look for a locally-owned fish store in the area since they tend to provide the most accurate information and high-quality fish. Quality pet stores will have compatibility charts for freshwater and saltwater fish.

This is an important step as selecting incompatible fish will have significant consequences. You could end up with stressed, harassed and colorless fish. Eventually, the fish that is not the alpha will die and you would have lost your investment.


Compatibility Chart

Ideally, you should select community fish as these fishes can get along quite well with one another. Mixing community fish is safe and will not create a tension-filled environment. You must avoid pairing community fish with aggressive fish as this will create an inhospitable habitat. Also, be mindful to purchase fish that are easy to take care of, if you are a beginner. If you are inexperienced, do not get fish that are only recommended for intermediate or experienced aquarium owners. You would end up overwhelmed with maintaining your aquarium.

Guppies, platies, swordtails, or mollies are good starter fishes. The size of your tank plays an important role. If your tank size is 5 to 10 gallons (18.9 to 37.9 liters), you might get 3 to 4 African dwarf frogs or possibly a betta and some shrimp.

Make sure that you conduct enough research and seek an expert’s advice before adding any fish to your tank. You should gradually introduce new fish to your tank. You can introduce a new group of fish every 2 weeks as well as adding the largest fish at the end.

Saltwater Aquariums

Saltwater Aquarium

Saltwater aquariums are also known as marine aquariums. A marine aquarium can house a wide variety of stunning fish and invertebrates such as starfish, corals and eels. Typically, exotic fish and invertebrates cost more than tropical freshwater fish.

Marine fish are more sensitive to water conditions and require a certain level of salinity for the fish to survive. Additionally, the salt must be purchased and mixed beforehand. To maintain the level of salinity, you may want to consider purchasing a salinity tester which measures the salt content of your marine aquarium.

If you intend to include coral in your marine aquarium, you need to purchase specialized marine aquarium equipment which is pricier. Be mindful that some types of marine fish require specialized diets which also adds to the cost of owning a marine aquarium.

Selecting Marine Fish

Compatibility Marine Fish

With a great diversity of color and form, marine fishes are one of the most popular of aquatic pets but require careful handling to avoid potential problems. One of the most convenient aspects of keeping marines is the fact that unlike many tropical freshwater fish, which differ in their environmental requirements, most marine species are found in similar conditions in the wild.

This means that fishes that are oceans apart in the wild state may be kept alongside one another in the aquarium. Nevertheless, you must keep in mind other significant factors such as territorial behavior and dietary requirements.

Unlike freshwater fish, marine species kept in the aquarium are almost always aggressively territorial. This means that adults of the same species are as likely to fight in an aquarium as they would in the wild. Such territorial aggression puts a limit on population density, it is best if you select young fish for your aquarium as their collection has little impact on healthy reef fish populations. If the fishes are captive bred, they tend to be less stressed and less aggressive than their wild counterparts.

In brief, a well-selected community of fish should consist of a variety of colour and pattern, with similar levels of aggression and adult sizes. Individuals should be added in increasing order of aggression and species should be researched before selection to ensure aptness.

Reef Aquariums


A reef aquarium holds a marine environment that specifically suits the growth of coral. Typically, a reef tank displays live corals and other marine invertebrates as well as fish that play a role in maintaining the tropical coral reef environment. A reef aquarium requires appropriately intense lighting, turbulent water movement, and more stable water chemistry than fish-only marine aquarium.

Most corals cannot tolerate the levels of nitrates, phosphates and other toxins that most fish and invertebrates will withstand, thus, you need to ensure higher water quality. Given that there are various varieties of coral, this will affect the kind of lighting, salinity and space in your aquarium. You should conduct ample research into the type of coral you wish to include in your reef tank first.

Selecting Reef Fish

When selecting your fish and invertebrates for your reef tank, ensure that your chosen fish do not pick at corals or eat invertebrates. Also, these fish must be able to coexist with one another. Your selected invertebrates also cannot consume corals.

Due to the territorial nature of reef fish, avoid putting species with similar coloration or markings together in the same tank, as they will normally clash over living space. Members of closely related species can also be poor choices as they are likely to require identical niches and fight as much as two individuals of the same species. Also, do not mix species with the same feeding pattern. For example, some Tangs and Blennies can be aggressive towards other algae-eating fish.

Selecting Corals


The term “Reef Safe” means that the species of fish or other marine life form are relatively harmless to any coral or other live inhabitant in your tank. When searching online, you should keep an eye out for this important term.

The main objective of keeping saltwater coral alongside your fish, is to keep them alive and thriving. Your tank will show visible signs of healthiness via moving, growing, consuming the nutrients you introduce as food. It is important to note that the level of interaction your corals and fish have with other inhabitants in your tank is relatively high.

Light is an important consideration when selecting your corals for your fish tank. You must fully utilise every usable area of light in your tank. This means that you need to place corals throughout the rockwork exposed to light. You must be mindful that corals grow at different growth rates and patterns.  Some species of coral such as Montipora capricornis grow rapidly. This species of coral can create large shaded areas within the tank and deprive other colonies of species of light

Unfortunately, there is no coral and fish compatibility chart.  However, merely engaging in a few extra hours of research and asking questions on the forums can achieve the benefits received by the proper husbandry of the two.

What is Reverse Osmosis?

Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water by pushing it under pressure through a semi-permeable Reverse Osmosis Membrane. Reverse Osmosis and Deionization creates purified water. Many marine and freshwater fish keepers use reverse osmosis (RO) to produce high quality water as the membrane traps and removes 90% to 99% of the impurities from the water. The purified water can then be used in the aquarium, while the waste water which contains concentrated contaminants can be discarded or used for your garden plants.

Why use Reverse Osmosis Water for your Aquarium?

There are several types of fish which inhabit very different environments. For example, some species of fish prefer a soft and acidic aquatic environment. In contrast, other species of fish inhabit mineral-rich lakes or a saltwater environment.

In some cases, a country’s tap water contains too many contaminants which would severely compromise the lifespan of your fish. This is especially so in the case of soft-water fishes like the Discus and Dwarf Cichlids. Furthermore, it is too risky to use rainwater to fill your aquarium as cities with heavy air pollution will have more acidic rainwater with harmful pollutants. Thus, this poses a problem for aquarium enthusiasts.

By using purified water, the limitations of your local water can be removed. Additionally, you can recreate a full range of aquatic habitats within your homes. Reverse Osmosis units drive the water molecules through a selectively permeable membrane, resulting in purified water without the hardness and pollution found in rainwater and tap water.

Benefits of Purified Water

By using Reverse Osmosis (RO) purified water, you can avoid pollutants in your tap water entering your aquariums. Untreated water can lead to algal blooms as well as kill delicate corals. The nitrate, phosphate and silicate levels of domestic tap water can cause problems in any tank.

With purified water, you can then decide on the types of sea salts or ingredients to add to your aquarium’s water. Therefore, this enables you to recreate any aquatic environment from scratch. The use of “pure” water which contains no contaminants will eliminate any doubts about what may be in your water.


However, you should also be aware that having pure water alone can create an unstable environment in your aquarium. In pure water, there is zero carbonate hardness. The absence of a buffering capacity makes it vulnerable to drastic pH swings. The lack of minerals also creates a poor environment for plant growth, fishes’ survival and development of your aquarium’s microbial communities.

Therefore, RO water needs to be re-mineralised. This problem is easily solved by using a re-mineralising buffer and these are available for both fresh and salt water use, enabling the fish keeper to produce the correct water parameters for every kind of fish. You can even add some tap water or a water conditioner to achieve the desired hardness and pH.

Many stores can supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added. With the growing number of successful mini marine tanks popping up in homes all over the country, it can save time, effort and stress knowing that some of the challenging work has already been done for you.

Components of your Reverse Osmosis Unit

To better understand your reverse osmosis unit, here is a list of your unit’s key components and functions.


The type of membrane you use determines the amount of impurities that the Reverse Osmosis unit will remove. The semi-permeable membrane acts as an ultra-fine filter which strains almost all unwanted constituents from the main supply. This allows only water molecules to pass through. Some high-output units may feature multiple membranes, thus enhancing filtration.

Given its vital role in any reverse osmosis unit, the membrane is expensive. However, it is also easily damaged. Therefore, you must handle your reverse osmosis with care, otherwise you may risk damaging it beyond repair.


Pre-filters are placed before the membrane and are quite essential. These pre-filters remove sediments, chlorine and other components of your main source of water. The removal of these components prevents any potential blockages and the destroying of your expensive and delicate membrane.

The pre-filters also need to be changed regularly to avoid any instances of failure. Otherwise, this will adversely affect the efficiency and lifespan of your unit’s membrane.

Flow Restrictor

This valve allows pressure to build up in the system. For reverse osmosis to take place, pressure is essential. Most reverse osmosis units require a minimum pressure of around 2.8 bar (40psi). At the same time, the ideal pressure will help prevent the system from leaking or blowing apart.

Your reverse osmosis unit may also have a pressure gauge. This allows you to monitor the pressure of the main water entering the membrane. Insufficient pressure reduces the unit’s efficiency and can even stop the unit from working at all. While not all units have a pressure gauge, they can be retrofitted.

Flush Valve

Some models contain the additional feature of a flush valve which bypasses the flow restrictor. This rapidly rinses impurities from the membrane. Thus, this prevents scaling or fouling of the RO membrane and keeps the membrane clean.

Some recommend adding the flush valve as it improves the efficiency, output and lifespan of a unit’s membrane. Under heavy use, the membrane should be flushed daily. Some units do not feature a flush value; however, you need not worry as flush valves can also be retrofitted to most systems.

Processes in a Reverse Osmosis Unit

Process RO

Most of the RO/DI systems which are used for producing fresh water filter the raw water in either 3 or 4 stages. While there are specialized RO/DI units, most of the hobby grade RO/DI units are relatively similar as they all use 10-inch interchangeable cartridges.

First Stage

In the first stage water is passed through a micron sediment pre-filter that removes silt, sediment, sand, and clay particles as well as any rust particles and debris that is created in the tap water system pipes that might clog the R/O membrane.

Second Stage

In the second stage, the water is then passed through an activated carbon filter that traps minerals and contaminants such as chromium, mercury, copper, pesticides and other chemicals. It also removes chlorine which is often found in tap water. This is important as chlorine will shorten the life of the membrane as well as the lifespan of your tank’s occupants. There are now specialized carbon filters available that will remove chloramines, a mixture of chlorine and ammonia, which is also commonly used to disinfect water supplies.


RO/DI units have deionization (DI) as the third stage. In deionization, there are two types of synthetic resins used; one resin is to remove positively charged ions and the another to remove negatively charged ions.

Cation deionization (DI) resins remove cations, such as calcium, magnesium and sodium and replace them with the hydrogen ion. Whereas, anion deionization resins remove negatively charged ions, such as chloride and bicarbonate, and then replaces them with the hydroxide ion. In Deionization, the displaced hydrogen ion and hydroxide combine to form pure water.


If a deionization cartridge is being used as the third stage, the fourth stage is the membrane. As mentioned earlier, the membrane plays a key role as it removes nitrates, silicates, phosphates, and other compounds.

There are several types of membranes which can be used in reverse osmosis units. However, the most frequently used membrane is the Thin Film Composite (TFC) membrane. The TFC membranes can be damaged by chlorine, but the carbon pre-filter will circumvent this problem. The water which has passed through the membrane is sent to the storage tank. The water which is not forced through the membrane, waste water, is routed to a drain.

Setting Up your Reverse Osmosis Unit

Most buyers have few complaints when it comes to installing their reverse osmosis unit. In fact, it can be easily installed by beginner-level aquarists. Before beginning your installation, inspect your filter and check if there are any damaged or missing parts.

Colour-coded Lines

The red tube is the line that supplies water to the RO unit and it is called the supply line. The black tube is the drain line for waste water. You can simply place this tube in a sink or down a drain and allow the water to flow freely. The blue tube has your purified water, which is free of contaminants.

You will need to find a location that has a source for cold water and a drain. You must then connect the filter to the source water and run the waste water line to the drain. This can be connected in places like a laundry room, under or next to a sink or outdoors near a faucet.

Most units will produce approximately 4 times the amount of waste water. Thus, for each gallon of RO water produced, it will make 4 gallons of waste water. The waste water will come out through a black line attached to the system. The black waste line can be connected to an under-sink drain using a saddle clamp.

If you are hooking up your filter system outside, then it needs to be protected from the heat and cold. Hot temperatures can ruin your unit’s membrane and freezing temperatures can cause the housing to crack.


For most reverse osmosis units, you must install the unit’s sediment, carbon and membrane into their respective housing. When you purchase a reverse osmosis unit, it will come with a manual and installation guide. Some units will be pre-assembled. You can refer to online resources for a comprehensive guide on installing your purchased unit.


Turn on the water and flush the system with a couple of gallons. You should look out for any potential leaks. After shutting the water off, you can proceed to the installing the DI resin cartridge. Lastly, turn the water back on and start collecting your purified water.

You should not keep the first 5 gallons of water that your reverse osmosis system produces. The first 5 gallons of water will contain pollutants that must be flushed out from the system. Also, when you replace the filters, you must still flush the first 5 gallons of water too.

Most RO and RO/DI systems are simple to set-up and do not require experienced hands. While some set ups may be a little more advanced, most standard RO and RO/DI filter installations take only 30 to 60 minutes.

Top 6 Reverse Osmosis Units

  1. AquaFX Barracuda RO/DI Aquarium Filter
  2. Coralife Pure-Flo II RO Unit
  3. SpectraPure MaxPure MPRO Reverse Osmosis Unit
  4. Koolermax AR-122 6-stage RO+DI Aquarium Reef RO system
  5. 6-Stage Aquarium Reef Reverse Osmosis Water Filter System
  6. Aquaticlife RO Buddie 3-stage Reverse Osmosis System

AquaFX Barracuda: Strong purification, fast outflow, easy to set-up

AquaFX Barracuda Aquarium Filter

Best Features:

  • Strong Purification
  • Fast Outflow
  • Easy to set-up
  • Affordable price
  • Excellent customer service

The AquaFX Barracuda RO/DI Unit is a 4-stage RO/DI unit which produces ultra-pure water at a fantastic price. This OR/DI unit is perfect for freshwater and marine aquariums as it has plenty of solid features at a bargain. Users were very pleased with the strong purification in this RO/DI unit. After testing the filtered water with a Total Dissolved Solids (TDS) meter, zero impurities were detected in the filtered sample. The AquaFX has a faster outflow in contrast to many other reverse osmosis aquarium filter systems.  In contrast, the AquaFX can produce 1 gallon of reverse osmosis water in just 6 min and 45 seconds. That is relatively fast when compared to the portable or countertop versions. The unit has a solid build and the rust-resistant metal frame provides a sturdy mounting system for attachment to a wall. Another notable feature with this unit is its quick set-up time. The unit comes with very clear setup Instructions especially in assembling this system.


  • Short hose
  • High water-wastage ratio

Some users remarked that the hose was a bit too short especially if your tap location is too far away. If the hose is too short, then you will need to buy a longer hose separately which will add additional costs. Additionally, there is also the issue of this unit’s high water-wastage ratio. This unit needs about 4 gallons of water to produce 1 gallon of pure water. While this is not unusually high, it is still significant for users who prefer zero waste reverse osmosis systems.

This model is available in 50 and 100 gallons per day (GPD) units.

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Coralife Pure-Flo II: Good filtration, easy to set-up, multiple attachments


Best Features:

  • Good filtration
  • Easy to set-up
  • Multiple attachments

The unit uses a premium three-stage reverse osmosis (RO) units employ Sediment Pre-Filter Cartridge (Stage 1) for ultra-fine pre-filtration, Carbon Block Cartridge (Stage 2) for chlorine removal before reaching the Thin Film Composite RO membrane (Stage 3) to effectively remove up to 98-99% of hardness, heavy metals and toxins. The installation manual included is detailed and easy-to-follow. This reverse osmosis unit is also wall mountable. Additionally, it does not require a separate flush kit, which saves you from spending more. It also comes with key attachments such as clear filter canisters, cartridges and faucet attachments.


  • Trouble with installing the cartridge
  • Some leakage

Some users had trouble installing the RO cartridge. The cartridge housing is not see-through; therefore, it was difficult to tell if they were installing the cartridge correctly. Users could not tell if it was assembled correctly until the unit was completely assembled and then tested. A couple of connections on the unit may have some leakage at first. In that case, you may need to unscrew the connections and reinstall.

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SpectraPure MaxPure: Effective filtration, high capacity, sturdy build

SpectraPure MaxPure MPRO Reverse Osmosis Unit

Best Features:

  • 99% rejection film
  • High capacity of 90 gallons
  • Sturdy build and components
  • Transportable
  • Simple installation

The Spectrapure MaxPure MPRO Reverse Osmosis Unit efficiently removes water contaminants in 3 stages and with optional saddles and tubing, the unit can be tapped into existing plumbing. The unit comes with a 99% rejection film. This means that the SpectraPure MaxPure can remove sediment particles that are as small as 1 micron across. This is equivalent to less than 1/25,000 of an inch. This also means that this unit can filter out some types of bacteria, making the water even “purer”. The MaxPure can also filter a large volume of water as it can purify as much as 90 gallons per day. This will suffice even for the largest of home aquariums. The model itself is made of strong materials like a non-rust mounting bracket which keeps the filters in good condition. The sturdy build is also important for keeping the model in excellent condition. The use of a manual flush valve will help keep the membrane intact for a longer period. The system is also designed to be transportable as it is only 15x7x16 inches. In terms of set-up, the unit is straightforward as the instructions are easy-to-follow and the unit comes mostly assembled.


  • High water pressure
  • Needs a flow control valve

Users commented that they would have preferred if the unit included a flow control valve for the input port, given the unit’s higher price. Users also cautioned that a high-water pressure can cause a leakage and can cause one of the lines to detach. Therefore, users should also consider purchasing a pressure gauge to avoid this problem.

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Koolermax AR-122 6-stage RO+DI Aquarium Reef RO system: Great filtering, less water wastage, fast outflow

Koolermax AR-122 6-stage RO+DI Aquarium Reef RO system

Best Features:

  • Effective filtering
  • High capacity of 120 gallons
  • Fast outflow
  • Less water wastage
  • Array of key features included

The AR-122 comes with an array of top-notch features. This 6-stage system comes with a reverse osmosis unit, pressure gauge, all filters as well as installation hardware. The pressure gauge reads the operating pressure. If the operating drops by 15psi, this prompts users to change the bottom 3 filters. This unit also includes an auto-shut-off valve which means that when output is closed, the input will also be turned off. While other models have just 4 or 5-stage filters, this model is the only one of the few 6-stage filtering systems in the market. The 6-stage filtration system is very effective in removing all the possible contaminants from the aquarium water. This model also comes with a high capacity of 120 gallons, which is uncommon as not many reverse osmosis systems for reef tanks have such a high capacity. In comparison to other models, this unit has a faster outflow and less water wastage. The clean-to-waste ratio for this model is 1:3 which is lower than many of its competitors.


  • A bit of fiddling is needed to fix it

Some users felt that professional help is necessary when it comes to fixing this unit. This is especially required when the system shows any signs of leakage. Other than this minor inconvenience, there were very few complaints regarding the performance of this unit. After all, the set-up is relatively straightforward as the unit comes as complete and ready for installation. Since the filters for this system are high capacity, both the pre-filters and post filters only need to be replaced once a year.

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6-Stage Aquarium Reef System: Effective filtration, high capacity, manual flush valve

6-Stage Aquarium Reef Reverse Osmosis Water Filter System

Best Features:

  • Effective filtration
  • High capacity
  • Manual flush valve
  • Easy to set-up

The 6-stage filtration process allows the unit to filter out the different contaminants. With a high rejection membrane capability, you filter out many contaminants common in the aquariums. The quality of purified water from this reverse osmosis system can even be used for cooking or even drinking. This unit is pre-assembled which makes the unit very easy to install. The reverse osmosis unit just needs to be attached to the aquarium and switched on. This unit also features a high capacity of 100 gallons per day. This is large enough to cater to even larger aquariums. Users are pleased that this model comes in other sizes too. The system also comes with a manual flush valve which makes it superior to many other RO/DI system in the market. A manual flush valve increases the lifespan of the unit while also reducing possible leaks. This is a very convenient feature as you only need to flush the system every few months.


  • The GPD often starts at low rates
  • Fragile elbow construction
  • Out-of-stock

When you first start using this unit, the output will not necessarily reach 50 gallons or 100 gallons. It may take a few days for the unit to produce the desired number of gallons per day. Another concern was that upon receiving the unit, users noticed that the plastic inlet elbow construction was very exposed. This means that the components could be easily damaged during shipping. Additionally, this model is sometimes out-of-stock on websites such as Therefore, you may end up paying more for this unit on other websites.

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Aquatic Life RO Buddie 3-stage System: Good filtration, quick connections, very affordable

Aquaticlife RO Buddie 3-stage Reverse Osmosis System

Best Features:

  • Good filtration
  • Strong mounting brackets
  • Quick connections increase convenience
  • Easy to set-up
  • Compact size
  • Great affordability

The Aquatic Life RO Buddie 3-stage reverse osmosis (RO) system features unbelievable affordability and functionality, all in a very compact design. The model comes in two capacities, you can choose the one with 50 gallons per day or 100 gallons per day. The three-stage filtration process comes with an RO cartridge, carbon and sediment filters. The unit uses a Thin-Film Composite (TFC) Membrane RO cartridges in conjunction with Carbon and Sediment cartridges to effectively and efficiently remove harmful substances such as heavy metal ions, dissolved solids (TDS), chlorine and particulate matter. The cartridges are also up-front and easy to replace. The quick connections make it easy to remove and replace cartridges while the sturdy mounting brackets will hold the unit securely while mounted in a convenient location. The unit comes with the added feature of a wrench which makes removing the membrane housing cap even easier. Even with a 100 gallons capacity, the small footprint of the unit makes it easy to place under cabinets or in limited spaces.


  • Likely to experience leaking if not fitted properly
  • Higher water-wastage ratio

One potential issue is that you are likely to face leaking especially if the components are not fitted properly. However, these parts are already pre-assembled and require little additional work. Adding an additional cartridge will not take much effort and there is plenty of help/video guides available online. The average waste or flush water, as it flushes the membrane, protecting it from being clogged, to filtered water ratio for budget RO/DI systems is 4:1. This is significant especially if you prefer a zero-waste filtration system.

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I hope this article makes purchasing an RO unit a little easier. As a casual lobbyist, be sure to avoid units that have odd sized or proprietary filters as they are very expensive to replace and can only be bought from one source. These 6 reverse osmosis units will offer excellent performance at a very affordable price.

Additionally, keep checking the membranes of your reverse osmosis system. If they become clogged, you must flush the membranes or even replace them. Usually, you only need to replace the membranes once a year. Otherwise, you may risk damaging your reverse osmosis unit and cause it to lose its effectiveness.