[If his tail is disappearing as fast as you mentioned in your other post, it's definitely a severe case of fin rot and the M and M2 are the right way to go. He will still lose some fin for a couple of days until the meds stop all the bacteria. It's hard to watch and wait as his fins disappear, but M and M2 are the best things to catch 99.99% of any bacteria that cause this. I've never seen them make a fish worse, but occasionally they are too late or not strong enough to cure the betta.
We're going through a case of this right now. Wally had a spot on his tail on day and within a day and a half, his tail was half gone. He was a very sick fish - just laying on the bottom except to come up for air. After one full course of M and M2 he is eating well and back at the top of his water, but still not swimming too well. He is on his second treatment now (3 days into it) and you can see some signs of new growth on his fins. Once he is done with this round of meds, he'll go on a maintenance dose of Bettafix for a long time.
Bettas are pretty tough fish, but they really can't handle stress. Ours got sick when we added a filter which created current and removed all the Bettafix maintenance dose from his water with its carbon filter pad (not a smart move on our part!) Some bettas do well with filters, bubblers, etc. and some do not. Temperature is another critical part for a betta. Too much temperature change will stress the out. A small heater with a precise thermostat is a must even for small tanks. Younger bettas seem to handle changes better than older ones (just like people.)
Despite all the "experts" out there, this is what I've found with our betta:
1. Use M and M2 at the first sign of fin rot.
2. Bettafix, Melafix, and Pimafix are great for maintenance dosage and small, mild, slow-spreading fin rot, not rapid moving fin rot.
3. Clean water is a must, but changing water alone will not cure severe or advanced fin rot.
4. Aquarium salt, BettaSpa, and similar additives will help keep your betta healthy, but will not cure infection.
5. Bettas usually don't like currents so filters are not a good idea on small tanks. Frequent water changes are much better.
6. Temperature is critical and should be precisely regulated at 75F to 82F.
7. There are a million companies selling a million different "cures" and "magical elixers" but they all just want to make a profit.
8. Most local fish stores don't have a clue about bettas (they're not a major money maker for the stores.)
9. Above all, use common sense. Trust your instincts and do what you think is right for your betta.
10. Sadly, bettas don't live forever (average is 2-3 years, especially for LPS bettas) so no matter how hard you try, you won't be able to cure everything.
11. You can feel glad that you did give your betta a better, longer life in a loving environment for whatever length of time you have him as opposed to him dieing in some small bowl/cup, neglected on some pet store's shelf!
but the one that lasted longest was the one we tried organic mixes with. Try to use meds with your fish on a last resort?
