If you live in the states you should be able to find methylene for $6 at a good dedicated non rip-off fish store. Just put it in a glass goldfish bowl or tupperware with an airstone and make sure not to feed or else ammonia will get out of control fast. Usually takes a few days to clear up guppy fin rot (99.9% of the time FUNGAL) because they are playful fish and cant help but get hurt and their tails are so large there is not enough blood supply to the tail, methylene will kill fungus, and increase their Red blood cell count and oxygen levels, its really great stuff especially for Quarantine new "fancy" fish that are pretty weak when you get them from the store. Put them in a ten gallon with a heater and airstone and 5ml methylene and keep it warmish and a good amount of salt and guppies will pick right up they love it. A week of this plus antibiotic feed and mine are good to go after about a week. After that its just about avoiding stress to them. Just got to monitor ammonia closely in the filter-less Q tank or hospital tank. Lots of 50% PWC in a ten gallon, everyday if you want to. Helps them get used to our water as well.