The Water

Tropical fish are remarkably tolerant of a wide range of water conditions, and will almost certainly be happy in ordinary tap water, provided you leave it to 'age' for a day or two before introducing them to it. This ageing can be important, for it will allow harmful (for the fish) chlorine to escape from the water, and it will ensure that the heating and filtration systems are working properly.

There will be times when tap water will not be suitable for a particular situation. There are a small number of species of the killifish family, that prefer soft, slightly acid water. You might want to breed some of the more difficult egglayers, and soft water will often be all the encouragement they need. In these cases you can use rainwater gathered from a clean, unpolluted water butt, and mix it with tap water until you achieve the required water quality. Do remember that ordinary gravel and many types of rock will slowly dissolve in the water, making it harder. Use a special type of inert gravel if you wish to keep the water soft.

You can slightly acidify the water by soaking peat in it for a few days, or by making up a small nylon bag containing peat and placing it in the filter.

Fish do not like sudden changes in hardness or acidity, so use a water testing kit to monitor the exact chemical composition of the water. The best advice must always be to leave the water in a display tank alone: any changes you make are likely to be only temporary, and as such, can do much more harm than good.

Green water
Occasionally, the water might become green. This greenness is caused by the multiplication of algae and although it is a nuisance as it limits the visibility, it does not constitute a danger to the fish or plants.

The remedy is simple. Switch off your lights for a few days and thereafter reduce the amount of time you have the lights on until a clean water balance is maintained. Provided your plants are healthy, it is a problem you are rarely likely to experience. Algae growth on the glass can be removed by a razor blade. Careful positioning of the lighting should avoid this problem.

Evaporation
All tanks lose water steadily through evaporation, and this loss should be replaced regularly before it becomes too severe.

Debris that falls to the bottom of the tank does no harm and gives the tank an established appearance. It should not be allowed to become too thick, however, and if necessary it can easily be siphoned off, creating as little disturbance as possible.