Fresh water is used constantly by the marine aquarist, first when the aquarium is initially filled and then for ongoing routine water changes. Of course, an appropriate amount of dry salt mix is added. Fresh water is also used for evaporation top-ups.
On the face of it using tap water seems reasonable – but is it?
Tap water is prepared for human use, and is safe: children and adults use it for drinking, cleaning teeth, showering etc. There are regulations about the minimum condition the tap water should be in, and the water authorities adhere to this. But…
Tap water has levels of nitrate, phosphate, insecticides, heavy metals, chlorine, chloramines etc that the marine aquarist could well do without. In other words, tap water isn’t just pure water. The unwanted parts vary according to area. Some agricultural areas could have higher nitrate and insecticides etc, and other areas could well have higher levels of heavy metals and the like. Water companies tend to add chlorine and chloramines as a matter of course.
Seawater quality is very important in a reef system and also in a fish only system. By using tap water the aquarist is adding items that are known to be a potential problem – nitrate is an obvious example. There is a method that is easy to use and solves the ‘contamination’ problem easily.
This is the reverse osmosis (R/O) method. This is were a filter, the R/O unit, is connected to the tap water supply, the tap water slowly flows through the filter, and the purified water is collected in a bucket. It really is as simple as that.
Reverse osmosis filters come in a range of sizes, with varying outputs. The output is often given as gallons per day, and the aquarist needs to choose one that will produce enough water for a routine water change (the initial fill of the aquarium is usually ignored as this will normally only be done once. R/O water should be used though).
The filter has stages, and the first is normally anti-sediment, which protects the later stages from clogging. Then comes an activated carbon stage, where chlorine is removed from the water. This removal is important as chlorine degrades and eventually could spoil the R/O membrane. The R/O membrane is the final stage- the membrane is very fine, measured in microns, and will only allow pure water to pass. Once the water has gone through these three stages it will usually be 95 to 98% pure.
Some R/O filters have a fourth stage, where the purified water goes through a special granular mixture. This gives the water a final clean. The fourth stage is not essential, but it does further increase the purity of the water.
Not all the tap water that reaches the filter will be purified, about one part in five is. There is some small variation on this. The water that gets to the storage bucket is ready for use, but the other four fifths or so is discarded along with unwanted contaminants. This latter water could be collected for use in a garden or similar if wanted.
The purified water that is in the collection bucket is ready for salt to be added to prepare a routine water change, or for use as a top-up to replace aquarium evaporated water.
There is a further reason why R/O water should be used, and that is to do with the dry salt that most aquarists use. Commercial salts nowadays are of a very high standard because the manufacturers make every effort to produce a properly balanced mix without contaminants. In fact, many manufacturers trumpet this fact by adding wording to the salt pack such as ’No Nitrates, No Phosphates’ or similar. So why add contaminants by using tap water and negate, even partially, the purity of the salt?
R/O filters do not cost a lot and are readily available. All they require is a tap water supply and normal mains pressure. The manufacturer’s advice on maintenance, which doesn’t amount to much, should be followed.
By using R/O water the aquarist is ensuring as far as possible that the water that is put in the aquarium is of the highest quality, and it is highly recommended.