Did you sterilize nets, tubing, filters, etc? Just checking. People tend to forget about nets.
You might try some garlic juice to see if that helps. It's kind of a "tonic," I guess. I only read about it recently, and it seemed to help when my betta was sick a few months ago. I'm not sure what the dosage is, or if it matters. You can get the juice by buying a small bottle of minced garlic at the grocery store very cheap. Also, there are manufactured bottles of it sold around the net somewhere for use with fish. Don't use melaleuca (tea tree oil,) even though it's marketed for bettas as Bettafix.
Temperature problems can majorly affect bettas. Try to keep it at least 75F. What size tank is he in? If it's tiny, as it looks in your avatar photo, wastes can build up very, very quickly. Someone more knowledgeable would know better, but if it's a betta bowl, you should change some maybe every day? You should post on that; every day might be too stressful, too.
Also, I read recently that if a fish's gills get burned with chlorine or ammonia or diffused gasses in the water (if your tap water fizzes when it comes out of the tap,) when/if they heal, the gills might permanently stick out from the scar tissue that forms. Perhaps it might be that? You need to let your change-water sit to de-gass, make sure it's the same temp as the tank, and treat it for chlorine. You could also use bottled spring water if it's a small tank.
If you haven't changed the water in a long time, it might be a big shock to the fish to suddenly change to all clean water, so then you'd make more-frequent but smaller changes, like 20% of the water every day for a few days until you can start what will be your normal water-changing routine (again, post in the forum so more-experienced keepers can tell you what this should be.)
Hmm, hope that helps!
Last edited by mfskarphedin; 05-27-2010 at 06:43 PM.