[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.