API General Cure contains Praziquantel and Metronidazole.
Praziquantel is used to treat tapeworm in animals and fish, and is a safe medication to use.
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Metronidazole is an anti-biotic used to treat people with some types of internal infections, and it can be used to treat fish with certain internal protozoan infections. However, it should not be used unless the fish have a known intestinal infection that has not responded to normal fish medications.
Improper use or mis-use of medications like Metronidazole, can lead to drug resistant bacteria that can kill people, animals, birds and fish.
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In my opinion, API General Cure should only be used as a last resort on fish that are sick with a known internal infection, and it should not be used unless that is the case.