Using Reverse Osmosis Water in Saltwater Aquariums

Reverse osmosis water purification system.
DmitriMaruta / Getty Images

Over the last few years, marine (reef) aquarium science has made gigantic strides. It may seem ridiculous now, but the importance of what was (and wasn't) in the water you put in your reef tank wasn't fully understood. It was discovered that even trace amounts of some seemingly innocent elements and compounds could make or break a reef tank. Most tap water contains elements which are not in natural seawater and which can inhibit the health of marine animals. Many aquarists have found that using Reverse Osmosis water for top off water, rather than tap water ensures that harmful chlorine is also removed.

Since most marine aquarists do not have a supply of natural seawater readily available to them, most aquarists choose to create their own aquarium saltwater by mixing commercially available sea salts with the best quality water they can find. It has been found that using “pure” water (H2O) containing absolutely no contaminants eliminates any doubts about what may be in your water. RO/DI water has been found to meet this requirement.

Reverse Osmosis & Deionized Water

Reverse osmosis is a filtration method which forces water through a series of filters, the last one being a semi-permeable membrane that removes 90-99% of tap water impurities. The result is water that is free of minerals and other contaminants, such as chlorine, chloramines, pesticides, nitrates, phosphates, and heavy metals. The contaminants are physically too large and are thus unable to pass through the pores of the system.

Most of the RO/DI systems which are used by aquarists for producing fresh water for saltwater aquariums 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 pretty much the same, using the same size (10") interchangeable cartridges.

First Stage

The 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

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 important, as chlorine will shorten the life of the membrane as well as your tank occupants. There are now available specialized carbon filters which will remove chloramines (a mix of chlorine and ammonia), which many municipalities now use to disinfect their water supplies.


RO/DI units have the DI as the third stage. In Deionization two types of synthetic resins are used, one to remove positively charged ions (cations) and another to remove negatively charged ions (anions). Cation deionization (DI) resins remove cations, such as calcium, magnesium, and sodium and replace them with the hydrogen (H+) ion. Anion deionization resins remove anions, such as chloride, sulfate, and bicarbonate and replace them with the hydroxide (OH-) ion.

In Deionization, the displaced H+ and OH- combine to form H2O.


If a DI cartridge is being used as the third stage, the fourth stage is the membrane which removes nitrates, silicates, phosphates, and other compounds. There are several types of membranes which can be used in RO units, but the most frequently used one is the Thin Film Composite (TFC) membrane. The TFC membranes can be damaged by chlorine, but a good carbon filter (stage 2) will eliminate this problem.

The water which has passed through the membrane is sent to the storage tank via 1/4" tubing. The water which is not forced through the membrane (dump water) is routed to a drain via 1/4" tubing.

Backflushing the Membrane

In order to extend the life of the membrane, it should be backflushed on a regular basis. Under heavy use, the membrane should be backflushed daily.