My fish is in a 5gal tank with plastic&live plants, a heater at around 80F, a filter, 2 corys and just gave it a 100% change today. A couple weeks ago, he stopped eating entirely. I tried giving him treats and cleaning the tank more often, but I only just today got him to eat something. Not as much as his normal (unsatiable) appetite, but something. Now he appears to be glued to the heater. That is, instead of swimming around and around like he normally does, he won't leave that corner, right next to the top of the water. He's been like that for a couple of days now and it got to the point where I got out the fish net and chased him around just to see him move. It's starting to worry me. What can I do?