For how long? Salts shouldn't be used in a tank for longer than a week to 10 days max as they can damage internal organs.
Anything that ends in "fix" can harm your betta, as this brand of medications contains an ingredient that coats the labyrinth organ in a betta (or the air breathing organ). Thus, slowly suffocating your loved one.
I would consult Sakura8 before dosing amoxycillin. I do not know how to dose this, as this is one of the medications I cannot work with due to allergies to the level of anaphylactic shock.