ive gotten away with only one or two treatments before, but it is probably better to do three just to be safe. to the OP, i too had a hard time getting interceptor without taking my dog to the vet first. i finally found one that i just explained my situation to. they had pity on me and gave me one pill. since one pill will treat 400g i was happy with it. just be sure to reassure the vet it IS for a fish tank, as weird as that sounds to them and good luck.