Making Reverse Osmosis Water Suitable for Aquariums

Why is Re-mineralisation Necessary?


While reverse osmosis and de-ionization filtration removes excessive levels of nitrate, phosphate, heavy metals and chlorine that aquarists do not want in their water source, it can also remove some of the essential elements. Therefore, reverse osmosis water must be buffered and “re-mineralized” before being used in any aquarium. Otherwise, the lack of minerals creates a poor environment for plant growth, fishes’ survival and development of your aquarium’s microbial communities.

This issue can be 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. Many stores can supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added.

Minerals in your Water

Before you make any adjustments to your reverse osmosis water, it would be useful to understand the different components of water. A stable environment is key to ensuring the long-term health of our valuable marine pets. These are some of the key factors that affect the suitability of your aquarium:

  • Potential of Hydrogen (pH) and is the measure of how acidic or alkaline a solution is. At a molecular level, this refers to the concentration of hydrogen ions (H+) in a solution. Below pH 7 means that a solution is acidic while a pH level above 7 is alkaline.
  • GH refers to General Hardness and measures calcium and magnesium as well as certain other minerals. GH alone does not influence pH.
  • KH measures the amount of carbonates and bi-carbonates in the water. It is referred to as a buffering capacity as it prevents drastic changes in pH values. KH directly influences pH as the higher KH levels will reduce drastic pH changes.

Reverse Osmosis water has zero GH and KH as well as a pH from 6.2 to 7.5. Before re-mineralising your reverse osmosis water, you must conduct ample research into the water parameters required by your chosen fish.

You should also work towards closely imitating the water conditions of your fish’s natural habitat. You can refer to this link which elaborates on the water parameters for each kind of fish:

Conditioning your Water

It is crucial that you avoid any sudden changes to pH, GH and KH. Many species of fish and coral will be adversely affected by an increase or drop in pH by 0.3 and this can even prove to be fatal for some.

You should avoid chasing “perfect” values for your pH, GH and KH levels. It is very difficult, in practice, to tailor the exact details of your tank’s water parameters to the needs of your fish. This is because the tank requirements of fish may vary depending on their species and whether they have been wild-caught or commercially bred.

Therefore, you should instead focus on maintaining a stable environment and water parameters, without adding too many substances and agents to the water. This will greatly benefit the growth of your aquarium’s inhabitants. These guidelines would also help to reduce any unnecessary stress on your fish and corals.

Avoid Sodium-based Agents

Home water softeners that employ either sodium chloride or potassium chloride should never be added to aquarium water. These compounds strip essential calcium and mineral ions while increasing sodium to very high.

This can ruin the balance of minerals in your aquarium as sodium is only required in trace amounts for most fish. Elevated levels of sodium can severely affect osmoregulation in fish, especially in fish such as Loaches that normally prefer “softer” water. Some aquarists have even used baking soda, which is the buffer sodium bicarbonate. We would not recommend using baking soda because the purity is questionable and the dosing may be unreliable.

Re-mineralisation for Saltwater Aquariums


If you are serious about your reef tank, many aquarists advise upgrading to a reverse osmosis water purification system so that you can start with pure freshwater.  Once you have bought your sea salts, you can start mixing it with your reverse osmosis water.

Unfortunately, there is no standard amongst sea salt manufacturers as to how many pounds of sea salt to add to a given volume of water. This is because every brand is slightly different from each other.  You should clearly follow the instructions on the brand of sea salt you bought in terms of pounds of salt to add per gallon or 5 gallons of water to get the right salinity.

You should not mix sea salts in your aquarium as the salts do not dissolve quickly and it takes a lot of checking and time to figure out if the salinity is correct.  Unless your tank is completely empty then you can mix the sea salts in the tank. If not, you should mix your sea salts with the reverse osmosis water in a bucket.

Here are some guidelines to mixing sea salts into your reverse osmosis water:

  1. Fill your bucket with a known volume water.
  2. Add a measured amount of sea salt. You can add 3 to 4 times more sea salt to the water in the bucket than the recommend final amount.
  3. The more sea salt you add, the longer it will take to dissolve. However, you will not have to mix as many buckets of mixed salt.
  4. For most saltwater and reef aquaria, the temperature should be from 76 to 80 degrees Fahrenheit. Do not worry about getting the temperature exact!
  5. Also remember that there is no one absolute correct salinity value. It is important to just get into a range. The salinity range should be from 1.020 to 1.024 on your hydrometer.
  6. If the water is too salty, you need to remove tank water and add more freshwater.
  7. If the salinity is too low, remove 5 gallons of tank water and transfer it to a bucket. Then, you can add some sea salts into the bucket.  The amount of sea salt to add depends on your situation. You will have to estimate and re-measure as you go along.

Overall, when it comes to adding minerals to your reverse osmosis water, there are no universal measurements of agents to add as each brand is different. Instead, the process of re-mineralisation works on a trial-and-error basis. You should focus on keeping to a range of values as specified by the company’s brand as well as avoid adding too many buffering agents which could undermine the stability of your aquatic environment.

How does Reverse Osmosis work in Aquariums?

In aquariums, there are typically 3 to 4 stages in reverse osmosis and de-ionization units. While there are specialized reverse osmosis/de-ionization units, most budget reverse osmosis units are simple to use. Most reverse osmosis/de-ionization systems are simple to set-up and do not require experienced hands. While some set ups may be a little more advanced, most standard reverse osmosis filter installations take only 30 to 60 minutes.

Process of Reverse Osmosis

Most reverse osmosis and de-ionization systems filter the supply water in either 3 or 4 stages to produce fresh water. The first stage of reverse osmosis reverses unwanted, large particles which could clog up your system. Afterwards, the supply water is passed through a carbon filter which removes chlorine and other minerals.

Flow of RO Unit

Sediment Pre-filter

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. These large particles are commonly found in your tap water system pipes. These particles might clog and damage your fragile reverse osmosis 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.

Carbon Filter

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.

This filter also removes chemicals that cause displeasing odours, cloudiness, colours or tastes to the water. For example, this could be hydrogen sulphide or chlorine. This filter also removes volatile organic compounds from the water.

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.

Reverse Osmosis Membrane

The membrane is expensive, yet fragile. It plays a crucial role as it removes nitrates, silicates, phosphates, and other compounds to produce pure water. There are several types of membranes which can be used in reverse osmosis units.

The most frequently used membrane is the Thin Film Composite (TFC) membrane. The membrane’s top layer is permeable to water and impermeable to various dissolved impurities including salt ions and other small, unfilterable molecules. The water molecules that pass through the membrane are sent to the storage tank. The water which is not forced through the membrane will be sent to a drain.


Most units also have de-ionization as the last stage. In de-ionization, there are two types of synthetic resins used to remove positively charged ions and negatively charged ions. Cation de-ionization resins remove cations such as calcium, magnesium and sodium and replace them with the hydrogen ions. Whereas, anion de-ionization resins remove negatively charged ions, such as chloride and bicarbonate, and then replaces them with the hydroxide ion. The displaced hydrogen ion and hydroxide then combine to form pure water.

Pressure Gauge

Pressure gauge

For most reverse osmosis units, you need a constant pressure of above 35 PSI. If your pressure falls under 35 PSI, you should consider purchasing a booster pump for your reverse osmosis unit as you need at least 35 PSI supply pressure to permeate the membrane.  If your pressure gauge drops below the 35 PSI range, it could be a sign that you need to replace the sediment and carbon filters. At the same time, you should avoid using a supply line of over 60 PSI as this may damage your membrane.

Flush Kit

Flush Kit

You can consider purchasing additional accessories to extend the lifespan of your unit such as a Flush Kit. A membrane Flush Kit extends the life of a membrane by rinsing debris and stop the accumulation of scaling. Some units have a built-in flush valve; however, it will not cost you much to purchase a separate Flush Kit if your reverse osmosis unit lacks a built-in flush valve.

Installing an RO Unit

Most buyers have little trouble 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. If you detect any problems, send it back!

Supply Line

There are 3 key lines that you need to connect before your aquarium’s reverse osmosis system is ready to go. Reverse osmosis units usually come with a detailed manual, specifying the function of each colour-coded line.

One tube supplies water from your tap supply to the reverse osmosis unit and it is called the supply line. The source of your supply water can come from the tap. Your supply line can be connected in places like a laundry room, under or next to a sink or outdoors near a faucet. If your aquarium is outdoors, it needs to be protected from both hot and cool temperatures, otherwise you risk damaging your reverse osmosis unit’s delicate membrane.

Drain Line

The other line is the drain line where your reverse osmosis system’s waste water is routed. You can simply place this tube in a sink or down a drain and allow the water to flow freely. Most units will produce approximately 3 to 4 times the amount of purified water. This means that for each gallon of reverse osmosis water produced, it will make 3 to 4 gallons of waste water. The waste water line can be connected to an under-sink drain using a saddle clamp.

Product Water Line

The last tube contains your purified water, which is free of contaminants, and sends it to your storage tank. However, if this is your first time using the reverse osmosis unit, you should not keep the first 5 gallons of purified water that is produced. 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.

After going through all these processes, your reverse osmosis unit is ready to use!

The Benefits of Reverse Osmosis Water

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. 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. This process can remove many types of ions and impurities from water that you would harmful to a home aquarium.

Most of the reverse osmosis and de-ionization systems produce purified water by filtering the raw water in either 3 or 4 stages. 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 not Tap Water or Rainwater?


Most tap water sources contain dissolved salts, nitrates and phosphates as per federal guidelines on city water treatment.  While this is safe for human consumption, it can be very unsafe for your aquarium. In fact, areas with excessively hard tap water will hurt the development of your soft water fish.

If your tap water contains even traces of phosphate, you are promoting algae growth every time you top off the tank. You may also deal with high chlorine and chloramine in tap water. Also, your tap water quality can change with the seasons, weather and chemical requirements which makes tap water inconsistent. This makes keeping a reef aquarium quite difficult.

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. While some people still use rainwater, this can carry harmful pollutants that may endanger your fish. Thus, rainwater poses a problem for aquarium enthusiasts.

Why use Reverse Osmosis Water?

Reverse Osmosis water has several advantages over tap water. Unlike tap water, reverse osmosis water is much purer and is low in organics, dissolved minerals and nutrient compounds. This removes contaminants which promote the growth of nuisance algae. Using reverse osmosis water also removes the limitations of your local tap water so that you can maintain a range of fish from different aquatic environments.

Reverse osmosis water and de-ionization units are very popular among aquarists. Reverse Osmosis and de-ionization are different processes which are often done in conjunction with other filtration systems. When these two processes are combined, pure water is produced.

Bypass Purification Chemicals and Cleaning Filters


Some users resort to using purification chemicals and cleaning filters to remove dissolved salts and particles from their tap water. However, this is a costly method and it may be cheaper to purchase a reverse osmosis unit instead.

Some users use filters and chemicals to filter tap water. However, this alone may not make the water safe for use in aquariums. The unit, by itself, cannot remove chlorine and chloramine compounds used in tap water purification. This could adversely hurt the development of your fish which are sensitive to chlorine.

Whereas, in reverse osmosis, particles are removed by a fine sediment filter and passed through a carbon block to remove chlorine. The water is then passed through the reverse osmosis membrane and removes nitrates, silicates, phosphates, and other compounds. Lastly, in deionization, water is passed through the deionization resins which remove minerals and compounds. This produces pure water.

Stable Environment

General Pic

Most aquariums require a stable environment such as a constant pH and temperature. A stable environment is essential for a thriving aquarium. As fish and coral cannot survive drastic changes in pH and salinity, reverse osmosis water offers the best possible water quality and stable environment for your aquarium’s inhabitants.

Through re-mineralisation, reverse osmosis water can be retrofitted to fit any aquatic environment. With reverse osmosis water, any fish keeper can produce the correct water parameters for every kind of fish. You can add a water conditioner or re-mineralising buffer to achieve the desired hardness and pH for your saltwater or freshwater aquarium. In fact, many stores supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added.

Easy to Install

AquaFX Barracuda Aquarium Filter

Most reverse osmosis units are pre-assembled and easy to install. Budget reverse osmosis filtration systems require little effort to set-up and can be used almost immediately for your aquariums. For example, the AquaFX Barracuda RO/DI Unit has a quick set-up time and comes with very clear setup instructions. Also, your reverse osmosis unit can be tapped into existing plumbing through the optional saddles and tubing included in the unit.

Most users do not require professional help in installing a reverse osmosis system. You will only need to connect the supply line, drain line and product water line. You just need to find a location that has a source for cold water and a drain. You can then connect the filter to the source water and run the waste water line to the drain.

Water Efficiency

Many reverse osmosis systems come with an automatic shut-off valve which means that when output is closed, the input will also be turned off. The diaphragm valves work in conjunction with a pre-pressurized tank to automatically turn the reverse osmosis system off when the tank is full, then turns back on when the tank level drops below a certain pre-set point.

You can also purchase a zero-waste reverse osmosis system. This means that for every gallon of purified water produced, there is zero waste water generated. Users who prefer a water efficient aquarium will enjoy a zero-waste reverse osmosis system.


A reverse osmosis system offers a simple solution for making any tap water source safe for your aquarium. By using Reverse Osmosis (RO) purified water, you can avoid pollutants in your tap water entering your aquariums. These pollutants can cause nuisance algae and long-term problems for the development of your fish.

Additionally, by using purified water, the limitations of your local water can be removed. Through re-mineralisation, you can add crucial minerals to your reverse osmosis water to aid the growth and survival of your fish. With reverse osmosis water, you can also establish stable water parameters to recreate any aquatic habitat within your homes.


Is Reverse Osmosis Water Suitable for Aquariums?

General Pic

What is Reverse Osmosis?

Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water to create purified water. It is a process of pushing water under pressure through a semi-permeable Reverse Osmosis Membrane. The membrane will allow the water molecules through, but will block larger molecules such as minerals and other contaminants.

Many marine and freshwater fish keepers have started using 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.

Reverse Osmosis Water vs. Tap Water

Many aquarists have switched to using water from reverse osmosis systems instead of tap water. In most cases, a country’s tap water supply contains too many contaminants which would severely compromise the lifespan of your fish.

Problems with Tap Water


Water from the tap contains a certain number of dissolved solids in the form of salts, phosphates, nitrates with varying concentrations of chloramines and fluorides. While the chemicals added makes tap water suitable for drinking, it can be toxic for your fish. This is especially so in the case of soft-water fishes like the Discus and Dwarf Cichlids.

Due to the nitrates and phosphates, nuisance algae grow will thrive.  While phosphates and nitrates are not harmful to people at lower levels, it can cause a lot of trouble in an aquarium even in very low concentrations.

If you plan on having a reef aquarium, tap water can wreak havoc on your invertebrates and corals. Trace amounts of copper and considerable amounts of total dissolved salts (TDS) can be toxic for your aquarium’s invertebrates and corals. In tap water, your TDS will be in the range of 250ppm or higher which fish and corals are not used to living with these levels of TDS in the wild.

Furthermore, as the water evaporates from your set-up, the TDS levels will increase and could become toxic over a very long period.  Also, the quality of your tap water may vary throughout the year.  This would make it very hard to make consistent batches of salt water and these changing water conditions can have adverse effects on sensitive fish and corals.

Choose Reverse Osmosis Water

Aquatic Life RO Buddie Three Stage Reverse Osmosis

In contrast, reverse osmosis water is free from all dissolved salts, minerals and particles. This is because of the process of Reverse Osmosis which drives water molecules through a selectively permeable membrane, resulting in purified water without the hardness and pollution found in rainwater and tap water. As such, reverse osmosis water has no hardness level and its pH is generally lower compared to normal fresh tap water.

One of the benefits of using purified water is that the limitations of your local water can be removed. Tap water may contain contaminants that can cause algae blooms in your aquarium. Therefore, by using reverse osmosis water, your aquarium will be free of any potential contaminants. Additionally, you can recreate a full range of aquatic habitats within your homes from scratch.

Limitations of Reverse Osmosis Water

One problem with reverse osmosis water in freshwater aquariums is that it may be too pure. It contains very few of the salts and minerals that your fish require for their environment.  The lack of minerals also creates a poor environment for plant growth, fishes’ survival and development of your aquarium’s microbial communities. Therefore, you should not use only reverse osmosis water. Instead, you explore ways to re-mineralize your reverse osmosis water. You should gradually experiment with different proportions to find out the water parameters you need.

Another problem with reverse osmosis water is that it has zero carbonate alkalinity (kH) and general hardness (gH). This means that reverse osmosis water has no buffering capacity and is very soft. Consequently, your aquarium will be vulnerable to huge swings in pH and other water parameters.

Most fish prefer a constant pH in the range of 6.4 to 7.8. They cannot live through large pH fluctuations, such as fluctuations of greater than 0.3 over a 24-hour period. Thus, huge changes in your aquarium’s pH levels will greatly compromise the health of your fish.

Therefore, you should always find ways to raise the kH and gH of your aquarium’s water. One way is through adding a commercial buffer or sea salts for a saltwater aquarium. However, when adding these products, you need to carefully monitor all the parameters to make sure you do not crash the aquarium.

Making Reverse Osmosis Water Suitable for your Aquarium

Remineralising agent

As pure water alone can create an unstable environment in your aquarium, you should explore ways to re-mineralise your aquarium. In pure water, there is zero carbonate hardness and a buffering capacity. This problem can be easily solved by using a re-mineralising buffer.

Re-mineralising buffers are available for both fresh and salt water use. This enables any fish keeper to produce the correct water parameters for every kind of fish. Many stores supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added.

Also, once you have decided to use reverse osmosis water, you should make it a point to constantly use it every time you top up or change the water as your pet fish will have already adjusted to these water conditions. It would be unwise to constantly use reverse osmosis water and suddenly switch to normal tap supply as this will place unnecessary stress on your fish as they will take time to readjust.


Overall, using reverse osmosis water in your aquarium has numerous benefits. With purified water, you can eliminate any nuisance minerals or salts from causing problems in your aquarium. With purified water, you can create any aquatic environment from scratch. This includes adding some crucial minerals to your reverse osmosis water is also key for the growth and survival of your fish.

You should also work towards establishing stable water parameters as they are key to a great set-up. If you subject your fish or corals to unstable conditions, they will likely experience at least some stress as they are getting used to the new water parameters.  In the case of severe fluctuations, you could experience losses with sensitive corals, invertebrates and fish.