Deteriorating dorsal fin on a Betta

Question
Hi...

My name's tony. I have a betta fish in a 2.5 gallon tank that I would change 75% of the water in on a weekly basis until I read/heard that that might not be too healthy for them, so I decided to go to once a month.  

Mr Chef (my Betta) sits on my desk at work and on the 2nd week of the month with no water change I noticed his big pretty dorsal fin had deteriorated quite noticeably and he was behaving strangely.  Hinding under the little toys and stuff I put in the tank to keep him entertained.  

I have three apple snails in there to keep the tank clean and cause they're cute as buttons, and I doubt they'd be able to get at him, besides they'd been living together for a few months before Mr. Chef's fin started to go.

How can I stop his fin from disintegrating, and will it ever grow back?

Answer
Ok this sounds like a case of fin-rot you should treat it with melafix and that should cure Mr. Chef. With the water changes you should do them once a week and replace about 25% of the water and before you add the new water put stress coat in the tank so the harmful chemicals are removed for your fish. Also with bettas they like their temp. to be at 80 and if the temp. is dropping alot that is what could have caused the fin-rot to start. I would get a heater for the tank if you don't already have one so you betta heals correctly and can live a good remainder of his life.