Hi, my friend gave me a baby Betta for my birthday last week and I got him a heater and a filter for his tank. He was being very active and fun for the first few days but now his fins have gotten very thin and he's become lethargic. He hangs out right next to the heater most of the time or at the bottom of the tank rarely moving unless I tap on the glass. He seems to be eating less and less as well.
When he was feeling good his fins were always waving around, now they are very thin an pinned back.
Here's a picture of him: http://cl.ly/image/1d19320n1p2T
Any help as to what I can do? Is he doomed??