It shouldn’t be necessary to change the water at all, providing the aquarium is balanced. I trust by now the word ‘balance’ has recurred a sufficient number of times under various headings for the would be aquarist to understand that it is, in my view, the operative word throughout tropical fish maintenance.
There are some conditions, nevertheless, when it is essential to change the water wholly or partially. Some aquarists prefer to change 10 per cent of the water weekly. I have experimented up to 50 per cent, but discovered no definite benefit.
It should be remembered that when water evaporates from a fish tank, the minerals within the water remain behind and constant topping up with tap water increases the mineral content. Distilled water or clean rain water could be utilized with out this happening.