Test the tap water for ammonia, nitrite & nitrate. If you have chloramine in the water supply, the water company might have overdosed with ammonia and that will end up in the tank when you do a water change.
If you aren't sure whether you have chlorine or chloramine in the tap water, contact your water company by phone or check their website and see what they add.
If you can get some distilled or reverse osmosis (R/O) water, test that for ammonia, nitrite & nitrate. They should all be 0ppm from distilled or R/O water, and the pH of distilled water should be 7.0.
