I'd start feeding them all fresh garlic mixed in with their regular food from now on. Also is your tank planted? It really should be. Definitely only keep one male guppy as a stud or none at all - they kill each other in smaller tanks. I put garlic through a garlic press and mix it with frozen bloodworms and skinned peas and chopped parsley. Then re-freeze it into little cubes. Eggcrate works great for this. This is what I always feed my fish. They're all healthy and all together
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