White Stringy Poop in Fish

Alright, we’ve cleared up the internet’s favorite fish diagnoses: swim bladder, dropsy, constipation and mystical green peas, but what about that “white stringy poop?” Somehow, many internet diagnoses involve this mysterious phenomenon in fish and have linked it to internal parasites. Well, let us set the record straight…

White stringy poop is a poop with no food in it.


Most gastrointestinal systems work very similarly, fish, humans and other pets included. Your gut is responsible for absorbing nutrients and getting rid of wastes and undigestible material. In order to help move things along, our guts are lined with mucus-producing cells to help things slide through. Without this lubrication, you’d never be able to pass a single poop, swell up and die. Fish have the exact same cells in their guts! And although freshwater fish cannot become constipated (sorry, but we’ve already been over this – you’ll have to take your complaints up with osmosis), they still need a little extra help moving things towards the exit.

Most of the time, you’ll barely notice this mucus coating because of what your fish eats. The mucus is stretched thin and you’ll see a mush similar in color to the pellets you feed. If your fish has not been eating, you will only see the mucus. This is the “white stringy poop” in fish. Is this a sign of disease? Not really, it only means your fish isn’t eating. A lot of those internet diagnosticians will recommend fasting fish when they are sick, which sometimes, isn’t necessarily a good idea. When fish are sick, they need nutrients to swim and run their immune system. Yes, some diseases may require limiting food, but that decision is for your veterinarian, not “helpful” strangers on the internet.


Yes, the “white stringy poop” does look a lot like an internal parasite. But unless your fish have come into contact with any wild-caught fish, invertebrates or unsanitized décor items, the chance of them getting an internal parasite is slim to none. In the 7 years we have been in business, seeing over 500 clients, we have diagnosed it TWICE. The biggest different between an empty fecal cast and a parasite is that one is alive and the other is not.

If your fish’s poop decides to get up and dance, preferably against the current, you may have something to worry about. Since they are so light and easily pushed around by water flow, some fecal casts can give the appearance of movement, but watch them closely. If necessary, pull it out of your tank using a turkey baster and put it in a clear glass or cup. In the still water, it will be easier to tell if it can move on its own.

So, let’s put the “stringy white poop” clinical sign away. Yes, it is a clear indication your fish is not eating, but you should know that already if you watch them during feeding time. There is no medication we can give you to get rid of it, and it is a NORMAL function of your fishes’ digestion. And never withhold food unless directed by your aquatic veterinarian.

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  1. Jessie,
    Great post. I see those from time to time. I had heard something that vaguely indicated it was a problem sign, but wasn’t sure. I’m glad to hear that if seen occasionally it’s probably nothing to get too worried about. Happy Fish Keeping,

  2. “If your fish’s poop decides to get up and dance” 😀

  3. but what if they’re eating and this still happens?

  4. If your black molly is pregnant and having white stringy poo could that mean she’s almost ready to give birth

  5. Currently treating my betta for bloating however her housemate in a tank of ten has been sinking from the rear tail. Please help

  6. This is so helpful, thank you. I’ve been squinting at my guppys’ poop for far too long trying to decide whether I should be worried and honestly I didn’t think I should be based on how the fish are acting (normal, healthy, active, very very interested in the brine shrimp I just fed them) but when seemingly the entire internet is yelling “that means parasites” it was hard to relax.

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