Cold water fish and tank issue?

Sounds like you killed the fish from shock. One if you got the tank sunday and the fish monday you didn't let the tank cycle enough. Let it run like if there was fish in there for more than a day. 2, when you clean the tank, let it sit with the new water in there for a little bit, make sure the water in the tank is the same temperature as the water in the fish's holding tank. Gravel vacs also help clean tanks without doing a complete water change. Othertimes is best to do 25 to 50 percent water changes, this helps save the fish from shock. Only do full waterchanges if it's really nessicary, I only do it if there are things growing in the tank that shouldn't be or if my fish all catch a disease. If you cant let the tank sit and the water temperature in the tank is at a differen't temperature than the hold tank, then float the fish in a container or bag "like when you first get them at the store" this will let the fish get used to the new water temperature and be quicker for you to release them into the tank. "lol, my mom did that with my fish when I was young too, we were babysitting my teachers guppys, she accidently killed them before we had to give them back. Try explaining to your teacher why she has a new species of fish!"