Alright... First. Dont let some noob come in here and be all OMG, I HEARD IT WAS AWFUL THIS ONE TIME!
There's a ton of misinformation around out there... It all started from one thing, and spiraled out of controlled via rumors, hear say, all of that.
Bettafix is watered down melafix.
Melafix is tee tree oil (melacula oil), an antiseptic.
Melafix does not and cannot:
CURE fin rot
CURE anything, really.
It CAN, however:
Keep fins and wounds clean, since its an antiseptic.
Helps heal injuries a bit faster.
It can damage their labrynth organ, IMO.
There's another reason its so widely warned about, too-
That, and only that, really, is the biggest issue with melafix .
But see, I didnt say Bettafix.
Because its not an issue with bettafix. Betta fix, though still potentially damaging, cant really be overdosed unless you went NUTS on it, and put an ungodly amount in.
Its watered down to a very large degree.
The reason so many people freak out about bettafix is because of its relation to melafix and the issues an overdose can cause-- and OD on melafix is pretty easy, you need to be careful.
If you ever use melafix, use no more than 1/4 the dose recommended.
Bettafix can be used probably at normal dose, but if you really feel the need to use it, under dose it a bit for good measure.
In general, betta fix and melafix arent worth it. Just use AQ salt!
And that's my two cents :)