I would suggest an aquarium salt bath in a quarantine tank if you have one. Particularly because it worked GREAT for me! After about 3 days, you could CLEARLY see the results! I think an aquarium salt bath for fin rot works miracles! One other thing is that you have to change the water every 1-3 days (depending on how big your tank is) and dose the salt again. Do this procedure for 4-5 days or once the black tips of the fins are not there anymore. Keep in mind that you should have a heater but a filter is optional.(In my opinion)
Good luck and I hope the little guy gets better!

 

If one were to be dosing salt in conjunction with water changes, the only salt that should be added is for the water that's been changed, not the entire tank.

 

I would use salt and frequent water changes. Adding a bit of stresscoat wouldn't hurt either.

 

Thanks all! I'll keep up the water changes and use the aquarium salt as well. I can also add stresscoat since I have it--just wasn't sure if it was safe for bettas.

 

Stresscoat is a water conditioner but it had aloe in it for the fish 's slimecoat. Keeping the water clean should help the fin rot and the stresscoat will help in healing.

 

Y'all just knew you would hear from the anti salt lobby didn't you.
The magical powers of salt in freshwater, protection, healing and stress. Salt is often given quasi-magical powers connected with healing.
Salt can aid in the production of the slime layer, you'll be told. And so could many water-born toxins or irritants. The second common magical power attributed to salt is that it aids healing Salinity levels that approach brackish water would repress Saprolegnia-type water molds, which can attack necrotic tissue. That's not the active consideration, though, when folks say some salt added to fresh water aids healing.
There is another wide-spread misconception, that some salt in the water is easing the stress of osmotic pressure. What stress they're fish and do a vvery good job regulating their own osmotic pressure This misconception is actively encouraged by packagers of aquarium salt. You'll hear this old tale repeated so often that, if it came to a vote, it might be voted "true."
Glad I got that off my chest again

R

 

Yup

Salt is largely aquatic snake oil.

 

Based on those pictures I don't see fin rot. I personally think this looks more like tail biting. This would explain the rapid onset, but I don't know why his behaviour has changed.

 

I agree with LBF (who is, btw, the original LBF!)

Fin rot - look for dark, discolored or "smudged" edges.
Tail biting - look for U-shaped chunks missing with no discoloration.

Could he have gotten his fin caught in the filter? It almost looks like it was sheared off.

If it was a sudden event (such as being torn off), this could explain a change in his swimming pattern, if it affects his balance or ability to swim.

I would try feeding him another Omega One pellet in the evening, rather than a freeze-dried treat.... Since he's eating the two Omega One pellets in the morning but not the evening treat, it could be that he likes the pellets better. (Then again, I'm not a big fan of freeze-dried food, so I'm probably biased on this one.)

Personally, I would also stop using the Aquarium salt. It can cause fluid retention and bloating. (And if he's feeling bloated, he may not feel like eating as much.) If you determine he has aggressive fin rot (rather than tailbiting), then I'd opt for a different treatment than Aquarium salt.