I have a ten gallon freshwater aquarium that I fishless cycled when I got it about a year and a half ago. I had three male platys, three glofish, and two spotted cory catfish. My heater broke in the summer and I had no time to go buy a new one and it is really hot in the summer where I live so room temp kept the take at about 74 degrees. Recently it has been getting very cold outside and my tank's temp had been slipping down to about 69 degrees or lower. My fish didn't seem to be affected at all but I was worried about them so I ran out and bought an aqueon submersible aquarium heater, 50 watt. I set it to about 76 degrees, left it in the tank unplugged for an hour and then plugged it in and it got my tank to 76. All my fish started to group by the heater and just lay on the gravel. One of my platys looked really sick and about to die. I thought perhaps since there was more heat the oxygen levels were down so I set up an airstone and turned it on. Last night he died and my other fish didn't look good. One of my glofish died this morning and my others, by the way they look, are bound to die today. I have no idea what caused this please help!!!!