Fish amoxicillin is a very common medication when it comes to treating aquariums. Amoxicillin is a highly effective way to treat many conditions, including dropsy, fin rot, and a variety of bacterial diseases. Fish owners often use this medication to treat entire aquariums when just one fish appears to be suffering from a condition, because these diseases are highly contagious and can quickly affect all the other fish in the tank. You’ll need to administer Amoxicillin according to the package directions. For example, a 250-mg capsule might be used daily to treat a 10-gallon tank. Pay close attention to the instructions on your specific product to determine the correct dosage and frequency.