To be honest, a lot of betta keepers stay clear of Bettamax now. It's
really just a really low-grade antibiotic that is essentially useless.
If your betta is sick enough to need an antibiotic, Bettamax isn't
strong enough to help, and if he's healthy, then he shouldn't be
exposed to antibiotics at all.

Dragon, don't worry too much about it, your bettas will be much better
off with good clean water.