Yesterday I put in a 50w adjustable heater in one of my 10 gal tanks. There are 4 cories in there, and I've noticed that one of them is definitely not right. I had to do a lot of adjusting with the heater to get it in the right place, but from what my two thermometers say, the temp has not gone below 76 or above 82. I did a 50% water change today, and tested the water before. Everything was normal. The other fish are fine, but the one cory is on a plant that is stationary at the top of the tank. He sits with the front part of his body up. What did I do?