With most problems with fish, the sooner you get on to it with the correct med the better so you will find that importers and serious fish breeders will have a pretty good first aid kit ready to go (and usually with many meds you will not get from a pet shop such as antibiotics and treatments for parasites). Meds in powder, tablet or capsules are usually stable for many years. Tablets and capsules are the best because the amounts are known and make dosing easier.