Ammonia, nitrites and nitrates are all toxic to fish in varying concentrations. Dissolved Organic Compounds may also cause long-term health problems, and so should be kept within reasonable levels. As far as I m aware, there are no test kits available to measure DOC levels at this moment in time, but it can be assumed that DOC levels will be reduced along with nitrate levels as the result of a water change. Water Changes Should be routinely performed. People tend to have their own criteria for deciding when a water change becomes necessary.
Books also have their own opinions about water changes and schedules.
The amount changed will depend a lot on stocking levels, how messy the fish are, the original quality of the change water, etc. I don t really intend on going into how much, how often, what water treatments should be used, etc, as I ll be here for a month of Sundays, and I ll only be repeating what most books, magazine articles and experts have already stated. What I don t really find necessary is reverse osmosis, adding trace elements, filtering over peat, water softening pillows, dechlorinators, products containing aloe vera, etc. What I will say about preparing water for water changes, is to find out what condition the water is in before beginning any preparation regime. Water varies tremendously, whether it be tapwater, citywater, well-water, etc.