The fish acted aggressively because it was on its own. Rainbows need company otherwise they stress out. You need to keep them in groups of at least 6, preferably 10 or more.
The fish is a male Melanotaenia praecox and he is really skinny. If he eats a lot but stays skinny, then he probably has intestinal worms. Section 3 of the following link has info on treating fish with worms.
The fish's diet should consist of about 50% plant matter and 50% insect larvae, shrimp, fish and other foods.
They are a lovely fish when kept in groups and fed properly.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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