Andy Pierce
Member
I'm also inclined to blame overdosing the dechlorinator. Note the instructions: Use 1 capful (5 mL) for each 200 L (50 US gallons) of NEW water [emphasis mine]. Old tank syndrome is tempting to blame and may also be a factor, but unless you are dramatically disturbing substrate on water changes that would be a more gradual decline rather than a sudden killing upon a water change. I really doubt there is something inherently evil in your water supply. Certainly switching to the correct amount of Prime is easy to try. What else are you adding to the new water - is it only Prime and nothing else? Since you are only replacing 20% of the water on a water change as long as you have reasonably good water flow in the tank you can rule out 'shock' from either rapidly changing pH or temperature - the 20% new water won't really move the needle meaningfully on either of those.