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.