The dream of every aquarist would be water that comes out of the tap exactly as they need it for their aquarium, but this is seldom the case. In large sections of the country, the tap water will have a fairly high GH (General Hardness) and KH (Carbonate Hardness), with an associated high pH value. While this type of water makes great conditions for African Rift Cichlids, it is far from ideal for South American Cichlids and Tetras. Or maybe the tap water has high levels of phosphate and maybe even nitrates that need to be removed before the water is used in the aquarium. The use of a reverse osmosis unit or a de-ionizing resin is the best option to remove these types of chemicals in the water. This allows the hobbyist to adjust the water to the parameters they require. Reverse osmosis units first found favor with breeders of Angelfish and Discus. Aquarists found that they had the best results with these fish if the aquarium had very soft water. These aquariums would have a GH less than 5, KH less than 5 and an acidic pH value, between 6.0 and 7.0. More recently, the use of de-ionizing resins has also been available to adjust the tap water. This article will present an overview of what is required to use these treatments to alter the tap water.

At the heart of the reverse osmosis unit is the reverse osmosis membrane. This is manufactured by two methods. One is the CTA membrane (Cellulose Tri-Acetate) and the other is a TFC membrane (Thin Film Composite). For most aquarium uses, the TFC membrane has better rejection characteristics. It is designed to reject 90 to 99% of most minerals in the water. The reverse osmosis membrane relies on the use of a semi-permeable membrane and water pressure to work. Under no pressure conditions, water will diffuse across this semi-permeable membrane from an area of high concentration of water (few minerals) to an area of lower concentration (lots of minerals). By applying pressure to the water, this normal osmotic reaction is reversed to force the water through the membrane from low to high concentration, hence the name, reverse osmosis.

Ideally only water will be passed through the membrane into the “product” water, but in practice, some smaller molecules do also pass through, primarily nitrates, phosphates and silicates, if present. Essentially all of the GH and KH will be removed from the water. The pH of the R.O. water can be widely varied, but once a buffer is added, the pH will lock in at the buffer’s set point. Along with the product water, up to 3-5 gallons of “waste” water will be created for every one gallon of the pure R.O. water. Depending on the size of the membrane, it can produce between 0.5 to 4 gallons of R.O. water per hour.

The basic design of the “full size” reverse osmosis unit has two prefilter chambers and then the membrane housing. The first prefilter is a sediment prefilter, rated to remove larger particles from the water. The most common sediment cartridges filter to either 5 or 1 micron. The second chamber holds a carbon (block or granular) cartridge that removes organic compounds and eliminates any chlorine in the water. This is an important step, since any chlorine in the water can damage the TFC membrane. After passing through these two prefilters, the water enters the membrane housing. This has two outlet ports, one for the product/pure water, and one for the waste water.

The line for the waste water has a flow restrictor in it to reduce the flow rate and increase the water pressure in the chamber. The pressure will be related to the total pressure of the tap water, and works best if this pressure is between 50 to 70 P.S.I. If the municipal water pressure is too low it can be raised with a booster pump. The waste water is usually directed toward a drain, while the product water is collected in some type of container. The Kent Marine Float Valve Kit can be added to automatically shut-off the reverse osmosis unit when the collection reservoir is filled.

The RO/DI series reverse osmosis units are similar to the full size units, but add a final deionizing chamber. The product water comes out of the membrane housing and then goes into the de-ionizing resin. These resins will remove those compounds that passed through the reverse osmosis membrane. Since nitrate and phosphate can cause algae problems, the RO/DI series can be the best choice for aquarium hobbyists. Also, for the marine hobbyist, there is the Hi-S membrane version reverse osmosis unit. This is a TFC membrane with the ability to remove up to 99% of any silicates in the water. Silicates can lead to diatom blooms in the marine aquarium so are best eliminated.

Reverse osmosis units are designed to produce approximately 10-12,000 gallons of product water before the membrane needs to be replaced. The sediment prefilter and the carbon block should be replaced after every 1000 gallons. If the tap water has a higher concentration of either “particles” or chlorine, it would be a good idea to replace these cartridges every 5-600 gallons just to be safe. Any time there is a noticeable change in the flow rate of either the waste water or product water, it is a good idea to check the cartridges. It is best to keep a diary of R.O. water made to know when to change the cartridges.

RO/DI water is actually too pure to use directly in the aquarium, unless it is used to mix with one of the commercial marine salt formulas. It can also be used to top off for evaporation loss, since any minerals remained in the aquarium, only the water evaporated. For freshwater purposes, it will require the addition of some minerals (electrolytes) and some type of buffering system to prevent pH fluctuations. Initially the GH and KH should read about 0.0 degrees. Commercial products can be used to raise these levels to any desired concentration. Be aware that if one of the RO/DI reverse osmosis units has been used, it would be counter-productive to use a phosphate based buffer, better to use something like Seachem’s Acid Buffer/Alkaline Buffer combo.

De-ionizing resins can be used as a stand-alone water filter or as seen above, as the final stage in a reverse osmosis unit. When used alone, the resins will also remove the minerals involved in GH and KH, primarily calcium and magnesium. Most resin units will change color as they become exhausted, and can no longer remove minerals from the water. Depending on the tap water parameters, anywhere from 30 to 150 gallons can be filtered with a unit like the Tap Water Filter. The de-ionizing resins do not produce any waste water and can usually be used at a production rate of 6-10 GPH. The water is totally stripped of any minerals, so electrolytes and buffers will be needed for freshwater applications. When a marine salt mix is added to this water, the result should be perfect salt water for the aquarium. While this water can also make great water for cooking or coffee making, it should not be used for drinking water.

The use of either a reverse osmosis unit or DI unit can help the hobbyist by removing any “unknowns” in the tap water, as well as removing those compounds involved in GH, KH and pH. This will allow the water parameters to be determined by the products added to the product water. This is especially useful where the normal tap water parameters are just too far from the desired aquarium parameters.
If the GH and KH do not need to be altered, but other compounds (nitrate, phosphate, silicates and unknown metals/pesticides) need to be removed, the Kold Ster-il unit might be a better choice. This unit has a submicron filter bag, capable of filtering even bacteria and spores from the tap water. It also has a carbon block with a 0.5 micron sleeve that will remove organic compounds and chlorine from the tap water. The real workhorse of the unit is its container of molecular absorption discs that will remove heavy metals, organic compounds, phosphate, etc. This unit can be run at 4 gallons per minute, so it can be used as an “on-demand” water system unit. It has rugge
d PVC ¾” inlet/outlet fittings and is rated to give up to 5000 gallons of filtered water before the media needs to be replaced. Best of all is the fact that there is no waste water produced, all the water that passes through the Kold Ster-il can be used.

Given a basic understanding of the water parameters desired in the home aquarium, the use of a reverse osmosis unit, deionizing resin column or the Kold Ster-il can allow the hobbyist to alter the tap water conditions to better match those required by the particular fish they are keeping. Be aware that it will take a little advanced planning to have R.O. water available when needed. It might be a good idea to always have one of the Tap Water Filter DI units available for “emergency” water changes.

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