Unfortunately, stringy white to grey poop has several possible causes. You have to work your way through the possibles. I'd start with constipation by feeding adult brine shrimp (the only thing adult brine shrimp is good for). To be conservative you can treat with Praziquantel. This is found in common fish meds like PraziPro. In this way, two issues are addressed.
Excess mucus in fish feces: this will present as white or light colored, stringy fecal material that often hangs from the fish’s anus for a longer than normal period. There are a number of different causes for this, some benign, others very serious. Without access to a microscope, there is little that can be done to diagnose this issue effectively.
Idiopathic mucus feces: this fancy term is just to describe white mucoid bulky feces of no serious consequence, but of an unknown cause. Some suspicion is that this can be caused by changes in diet or diets high in fats. Very common in Clownfish.
Starvation: Fish that have no food moving through their bowels may excrete white mucoid feces with little bulk to them. The primary diagnostic for this issue is evident in that the fish won’t been seen to be eating. Resolving the anorexia is of course the prime focus to resolve this issue.
Bacterial infection: internal bacterial infections can cause stringy feces as well. While some of these may resolve on their own, medicated foods containing a broad spectrum, gram negative oral antibiotic may be required.
Metazoan infections: multicellular worms are often blamed for mucoid feces, but in reality, they are almost never the root cause for this, and really, can only be diagnosed through looking for their ova in fecal samples. Fish can harbor tapeworms and nematodes without producing mucoid feces.
Protozoan infection:
Hexamita and related diplomonad flagellate protozoans very frequently cause white stringy feces, especially in newly acquired clownfish. Metronidazole is the most frequent treatment for this issue. It can be dosed orally at 25 mg per kg of fish body weight, or 5 g in a kg of food. However, it is a very bitter medication, and some fish will refuse to feed on food containing it. It can be dosed in the water at 25 mg/l, but this is more effective in treating marine fishes, as freshwater fish do not “drink” aquarium water. I like to use
COOL MED Medicated Brine Shrimp Flake with 2% Metronidazole
medicated food from Brine Shrimp Direct.
Coccidia: These microscopic, spore-forming, single-celled parasites are common diseases in dogs and cats. In fish, they cause epithelial necrosis of the gut, enteritis and the copious production of mucus (Noga 2010). These cannot be diagnosed without microscopic examination, and there is no well researched cure for this in fishes, although Toltrazuril has been tested.
Constipation: This malady is often given as a cause for stringy feces, but it is not as common as one would think. When it is seen, the feces are usually dark, not light in color. Often touted as a “cure for constipation”, many aquarists add Epsom salts (magnesium sulfate) to the aquarium’s water – do not do this in marine aquariums, its use is primarily as a tonic/dip for freshwater fish. Seawater formulas already contain between 7 and 19 grams of magnesium sulfate per gallon (depending on the recipe). NSW magnesium level is around 1200 ppm and sulfur is around 840 ppm. Some benefit might be seen using it as a dip in additional concentrations, but adding a small amount to a marine aquarium itself has no benefit. Epsom salts do have some possible benefit when mixed into the food as a 3% by weight adjunct. Another commonly heard remedy for constipation is; “feed fresh peas”. This is an effective cure for fancy goldfish and Malawi cichlids that develop constipation and bloating. Again, overextrapolation has marine aquarists trying to feed peas to carnivorous fish, etc. The best method to enhance gut motility in fish is to feed frozen adult brine shrimp – not as a permanent diet, just long enough to get the constipation resolved.
