We picked up a fish tank today from someone on freecycle who was moving and couldnt keep them any more. Its a 20 gallon tank with several different fish, some gouramis (sp?) tetras, hatchet fish, a couple plecos, and a few more. She had drained alot of the water out for transportation and when we got home and added new water a few fish started to die. We lost a few tetras and a big pleco (he was my favorite). Could the new water be the cause of the dead fish?

Ive never owned fish and my mom owned fish when she was a kid (thing were alot different then) and we were told to just make sure the new water was the same temp and the water in the tank. Also the fish were all pretty much hagning out at the top of the tank for a long time, my mom thinks it might be from a lack of oxygen, so I added a tube bubble thing and thay all seem to be ok now, except for 2 of the gouramis (sp?) and the hatchet fish. Any help?