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!

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