Are you also doing any kind of maintenance on the filter(s) when you do the water changes? Are the filters turned off for any length of time? How do the fish seem after the water change? Are they breathing harder than normal or hanging at the top or bottom of the tank? How much water are you removing? Are you adding anything to the water besides the declorinator and what kind of declorinator are you using.
And lastly, (sorry for all the quesitons), what are your water parameters?
Robin
And lastly, (sorry for all the quesitons), what are your water parameters?
Robin