Parasitic worm that got lost in the fish. It happens sometimes when it rains. The rain washes animal poop into water ways and intestinal worms end up in the river. The fish pick up the worm eggs and the worm larvae appear in the fish. But because the fish is not the correct host for the worm, the worm gets lost in the body and can appear anywhere in the fish.
You can try deworming the fish. Flubendazole treats flat and round worms. Levamisole treats round worms. Praziquantel treats flat worms.
See section 3 of the following link for more info about deworming fish.
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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