It sounds like your fish is sick. Is your filter rated for a 20 gal tank? a 5-15 makes it sound like it rated up to 15 gal. Anyway, it sounds like you didnt get your tank fully cycled and your fish is suffereing from it. There are many ways to cycle it, depends on who you talk to, if you are going to cycle with fish, there are certain fish that will handle the cycle better. A hardy fish is needed to cycle with fish, otherwise you can cycle without fish.
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