Stringy white poop when the fish is eating well is caused by intestinal worms. Guppies are Asia are always infested with worms. Section 3 of the following link has info on worms in fish.
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What to do if your fish has Stringy White Poop.
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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