What you need to know about reverse osmosis


Ever more marine and freshwater fishkeepers use reverse osmosis (RO) to produce high quality water. Dave Wolfenden explains how RO units work and how to use them safely.

Several components comprise an RO unit. Let's look at the function of each one:


The semi-permeable membrane acts as an ultra-fine filter, straining virtually all unwanted constituents from the mains supply and allowing only water molecules to pass through. This part is expensive and easily damaged, and some high-output units may have multiple membranes.

Flush valve

Some models incorporate a flush valve which bypasses the flow restrictor. This allows deposits to be washed from the membrane, improving efficiency, output and lifespan. Flush valves can be retrofitted to most systems.

Flow restrictor

This valve allows pressure to build up in the system — essential for reverse osmosis to take place —while allowing water through above a certain pressure, preventing the system leaking or blowing apart.

Product water outlet

'Product water' is 'cleaned' tapwater for the aquarium and an efficient RO unit can remove 90% or more of tapwater contaminants.

Regularly test it with a TDS (total dissolved solids) meter or a GH (general hardness) test kit to check if the unit is working correctly. Product water should have a TDS of 10ppm or less and a GH of zero.

Waste water outlet

The waste water has elevated levels of nitrates, phosphates, heavy metals, pesticides and other nasties and shouldn’t be used in the aquarium. Don’t waste any of this water though, as your garden plants will love it!

Pressure gauge

This allows the pressure of the mains water entering the membrane to be monitored. Insufficient pressure will reduce unit efficiency, or prevent it working altogether.

Not all units have a pressure gauge, but they can be retro fitted.


Placed before the membrane, these are essential. They remove sediments, chlorine and other components of mains water which would rapidly block and/or destroy the membrane.

Our tapwater is subject to strict quality guidelines, but, even so, contains substances our aquarium’s inhabitants wouldn’t like. Tapwater quality and chemistry varies tremendously from region to region, but we’ll often find harmful nitrates originating from agricultural run-off.

UK tapwater is legally allowed to contain 50 mg/l nitrate.

Phosphates and silicates may also be present, which also act as algal fertilisers promoting nuisance algae. Other chemicals may be present, including heavy metals, which won’t be appreciated by sensitive marine invertebrates.

Reverse osmosis purifies tapwater to remove such contaminants. It relies on extremely fine membranes, made from Thin Film Composite (TFC) materials to strain unwanted dissolved chemicals, including nitrates, phosphates and heavy metals such as copper, leaving only water.

This needs to happen under pressure, against the osmotic gradient of the water — hence 'reverse' osmosis.

Normally, water would pass across a membrane from low to high solute concentration, but, in the case of reverse osmosis, the opposite happens.

The purified (or 'product') water can then be used in the aquarium, while the waste water, which has concentrated the contaminants, can be discarded or used for your garden plants.

The ratio of product to waste water varies, depending on various factors, but expect to waste around 5 l/1.1 gal of water to produce 1 l/0.2 gal litre of product.

Before the water can pass through the membrane, it needs to be prefiltered to remove sediments and chlorine which would otherwise rapidly destroy the membrane’s delicate TFC materials.

The prefilter media needs to be changed regularly, as failure will compromise the efficiency and life of the membrane.

The performance of any RO unit depends on several factors, notably temperature and pressure of the mains water.

Lower temperatures tend to decrease output and a minimum pressure of around 2.8 bar (40psi) is needed.

Lower and it’s unlikely that much product water will be made and, in such cases, a booster pump will be necessary.

However, most domestic mains supply pipes in the UK have adequate pressure.

Installing an RO unit needn’t require the services of a professional plumber. A simple screw-in saddle tap can be fitted to the coldwater mains pipe to supply the unit and a drain clamp can be fitted to any plastic domestic drainpipe to run away waste water.

Using RO

Using RO water is a doddle for marine aquarists. Simply use the product water to make up a salt mix and the neat RO can be used for freshwater top-ups without needing additives.

Many marine aquarists, and reefkeepers in particular, would consider an RO unit mandatory, as the levels of nitrate, phosphate and silicates in untreated mains water can lead to persistent problems with nuisance algae. If not already using RO for your marine system, seriously consider it.

Things aren’t as straightforward for freshwater aquarists and it’s important to appreciate that neat RO water can be lethal for fish!

Because it’s nearly pure water, it has zero KH (carbonate hardness).This means that its lack of buffering capacity makes it prone to drastic pH swings.

Its lack of minerals also plays havoc with fishes’ osmoregulatory systems, as well as creating a poor environment for plant growth and development of the biological filter’s microbial communities.

Pure water is too sterile and potentially unstable for aquatic life, but can be a ‘blank canvas’ for chemical fine-tuning to create just the right conditions for the aquarium’s inhabitants. If wishing to keep very sensitive soft water species, for example, RO is the way to prepare water.

RO water needs to be remineralised and there are two approaches. The first involves adding a certain amount of mains tapwater, preferably to which a water conditioner has been added, to RO water to achieve the desired hardness and pH.

Adding tapwater

It seems bizarre to add tapwater to RO after we’ve purified it. This method does, to a certain extent, dilute the contaminants in the tapwater and this can work just fine for, say, soft water systems needing only a low proportion of it.

However, it starts to make less sense if, for example, very hard water is desired, necessitating lower proportions of RO water and larger quantities of tapwater.

A more elegant method of remineralising RO water — allowing for greater control over water chemistry — is using specific remineralising powders or liquids.

The water needs to be prepared in a separate container, never mixed in the aquarium, and, with such products, it’s possible to create the ideal conditions for aquariums, ranging from soft, acidic river systems to extremely hard, alkaline Rift Valley lake biotopes.

TFCs need TLC

The TFC membranes in RO units make up a large proportion of the overall cost of the system. They are delicate and easily damaged, so look after them.

Regular replacement of the prefilters, plus regular flushing of the membrane, will help prolong its life.

Once an RO unit has been run for the first time never let the membrane dry out, as this will render it useless.

Seeking even greater purity

Some aquarists opt for an additional deionising (DI) stage through which the product water flows before being used in the aquarium. Such systems are referred to as RO/DI.

The DI resin removes additional quantities of impurities left behind following reverse osmosis, resulting in water virtually devoid of any contaminants.

For most aquarium applications, however, a standard RO-only system is perfectly adequate.

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