I'd treat him with myxazin for 5 days. It's a fungus and maybe finrot. Mine had the same recently. Go buy some Myxazin from ebay or your lfs. Do a 50% water change then dose with the myxazin. Treat his tank with the myxazin daily for 5 days then on the 6th day do another 50% water change and he should have improved by then.
Could you post a pic of him? Even if its a poor one so we can see what you're talking about. I still want to rule out parasites. Also you in UK or US? meds will differ depending.
cheers
I'm with Loraxchick on this, it doesn't look like finrot. Sorry to hear hes not doing to wee Dav
if you could get another pic of him that would help no end
the fungus on his fins could be a secondary infection due to his already weekend condition. i second adding salt as a general tonic rather than medicating for a secondary infection rather than the cause.
In all likelihood was not your fault. He probably was carrying some sort of infection when you got him. Im still convinced it was some sort of parasite so watch your other fish for symptoms such as flicking or rubbing against objects in the tank. rapid breathing/gasping, thick slimecoat or paling of color, spitting out food, getting bloated or skinny despite normal feeding habits, lethargy, etc. not to scare you but something to keep an eye out for.
There are ALWAYS nasites in the tank that can make our fish sick. Healthy fish can fight it off 99% of the time. Stress, harassment from tankmates or poor water can weaken the fish's immune system and their ability to fight off these things. My guess is he was not well when you got him
Better luck with the next boy.
all the best
cheers
Good luck with your Betta they are wonderful fish and a joy to keep, just keep a look out for anything, because as you have seen in my case it can just turn overnight.so sorry to hear about bully dav. but im going to buy a betta so ur forum really heped me for things to look at for
Thank you lilfishie, such a shame he was a beautiful fish, in a few weeks I will start a fish less cycle again and then be on the hunt for a new boy.so sorry to hear this Dav, i agree he must of been ill when you brought him