Some moron threw a bunch of Bettas in a tank together and they all
died but Ned. I have smuggled him out and want to know how to care
for his mangled fins. He seems otherwise active.
never cease to amaze me the #@&$ people come up with. How
ignorant can one be? Throwing a bunch of bettas in one tank is a
great recipe for complete bloody disaster. And I meant
Ned is very lucky that you have rescued him and that he escaped the carnage. He probably was the alpha male and that is the only reason he survived: He was winner of the battle (either that or he was the big coward who hid behind a rock the whole time LOL).
It will be vital to keep Ned's water very clean to prevent bacteria bloom in his tank/jar. Next you will need to add BettaZing to his water. Follow the manufacturer's instruction to figure out how much to put. I don't know how big a tank/jar Ned is in so I cannot help you on that end. Bettamax is a great disease preventative and also contains vitamins, all of which will greatly help Ned right now. Keep bettamax in there for about 3 weeks and keep a close eye on Ned's fin edges. If his frayed fins heal, you will notice after a couple of week a regrowth. This will start by looking like a clear edge. The fins grow back clear in many cases. As weeks go by the "edge" will get larger and look more like a ban, etc...
However, if instead of a clear edge you start seeing a dark or red edge, and the fins do not get longer, but instead might get shorter, that means his frayed fins are being attacked by a bacteria or fungus that causes fin and tail rot. Yikes! If that is the case, you will need to switch Ned to some heavier duty medication, see my page on disease for directions on how to treat fin rot. You will do a full water change, you will no longer put Bettamax in the water but instead you will put the other medications I prescribed on above mentioned page. You will keep Ned in the new medications for as long as needed. Some fin rot can be very stubborn and take weeks to go away so be diligent and don't give up!
Good luck to the both of you!