Aquarium Water Chemistry and Reverse Osmosis.
For most people, the water from the kitchen tap is mostly OK for a typical fish tank. However, water quality can vary greatly, even within the same county. This is one of the advantages of finding an experienced aquarist in your area – for most people this will be a local dealer. They will advise on the suitability of your local water. Note that most shops keep fish in local water, so that the water chemistry matches that of the average customer. This is why it is so important to acclimatise fish to a new environment. The water in the tank could be very different to that in the transport bag.
Tap water usually contains additives to maintain cleanliness of the water for drinking. This usually means traces of chlorine or flourides and are harmful to fish. If tap water is used, a product to render these substances inert should be added.
Water from the hot tap SHOULD NOT BE USED. The hot water tank is often made of copper which is particulaly toxic. Simply use cold water, and add it slowly.
Reverse Osmosis water (often called RO water) is water with all minerals and other impurities removed. It is sold by most aquarists, or is made at home by hard-core enthusiasts (the equipment to make RO water can be expensive). Minerals must be added before it is put in the aquarium: water that is totally devoid of minerals can quickly kill fish.
RO water is useful because it is less likely to contain trace toxins (such as heavy metals or chlorides) and formulating the necessary mineral additives provides an opportunity to precisely control parameters such as pH, KH and GH, so that water chemistry can be targeted towards particular fish species.
Aquatic plant enthusiasts use RO water because low hardness water can be fertilised with a greater concentration of carbon dioxide without becoming toxic for fish.