Ok, I've not killed anything, well, nothing bigger than 1 millimetre, but that yucky mucky water that I rinsed the filter sponges out in (after mixing up the gravel), well, I left it on the floor by my tanks last night, and forgot it was there whilst re-arranging the cables, I only went and spilled the stuff all over my floor!!! I was almost sick at the smell!!! I'm just glad I have a few Kirby vacuums to deal with the problem once it's dried out, if it dries out.... 8O
So, here's a good tip:
Throw used tankwater away instead of looking at the little creatures zipping about in it...