So here's the thing, I had recently bought 3 small catfish (I'm gonna be honestly I forgot the breed) not too long ago and put them with my two Danios. They were enjoying themselves quite nicely until today. I brought home 4 kuli loaches and put them in the tank (I have a 10 gal). Everything was fine, and I noticed the tank was a bit dirty, so I did about a 40% water change. And to my probably lethal mistake, I accidentally put about 2 gallons of water back in the tank without putting any de-chlorifier (sorry for my spelling) in it first. I assumed the fish would be fine since that had happened a long time ago with my old tank and everything was fine. So I finished putting the last 2 gallons in, remembering the de-chlorifier this time. Everything seemed okay for about 20 minutes, but then I looked back at my tank and the three catfish were swimming around very helpless looking. One kept it's nose to the top while the other two struggled around and barley moving at the bottom... A friend told me it could be the chlorine so I put some de-chlorifier in (the kind you don't need to let sit or mix; ie it was safe) and just hoped for the best. It was only a matter of 20 more minutes till the 3 catfish passed away all at once.... possibly the strangest thing I've ever seen. The other fish are completely fine. Now I'm very new to catfish and I feel awful about what has happend, so more or less I'm asking for an explanation of what may have happened? Was it the chlorine? Thanks so much, Gabby.