[masraum]

Originally Posted by tevake

Good idea Baz, I was helping a friend refresh the water in his tank, a large one that I had helped sort the plumbing for. And he decided to go for a bigger water change than recommended, Changed almost half the water at one time.
Next morning there were a lot of belly up fish.

Not sure what did them in, too much temp change, too much chlorine added at one time. It's a delicate ballance to maintain in a water change.
I think getting the new water close to the temp in the tank at least is a step to take.
Also some of the chlorine will off gas if left sitting out in an open bucket for a while.

Cheers Richard.

My mother used to change most of the water in the tanks that we had. She had gotten very good at it. She also eventually got up to three 20 gallon tanks. Her method was to take the fish out and put them in a bag, and float them in one of the other tanks. Then empty (mostly) that tank, clean, refill, do the chemical thing, then let the tank get to the right temp. Then she'd transfer the bag from one tank, to the other and let the bag chill in that tank for a bit. That allowed the water temp in the bag to change slowly to the overall temp and not shock the fish. Then she'd release the fish back into the newly cleaned tank. When changing water, you need to deal with the chemicals and the temp so as to not shock the fish. I'm assuming that one or both of those things got your friend's fish.