The correct term for this condition is prolapsed rectum.
Main causes are improper diet (which cause severe or chronic diarrhea, or constipation.), or bacterial and parasitic infestations that cause swelling and intestinal blockage. Once the cause is corrected and the anal sphincter muscles recover back to their normal strength, the prolapse should correct itself. In a rush to see larger kok size some hobbyists forget that FH have been derived mostly from CA cichlid species, that are for the most part omnivorous, not strictly carnivore.
FH require high quality plant matter in their diet just as much as they do fish/meat protein. (see scientific paper below)
Study of the causes of Anal prolapse in Flower-Horn Fish
Authors:
[ S Mehdizadeh Mood ] - Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran
[ H Ebrahimzadeh Mousavi ] -
Abstract:
Objective: Flower-horn fish is considered to be the result of cross breeding between the Cichlasoma trimaculatus and Cichlasoma festae. Several flower-horn fish were brought to the aquatic animal health department of Tehran University with a prolapsed anus. These fish showed listless, lethargy, and anus prolapsed while demonstrating minimal desire for food. Method & Materials: In order to identify the cause, a bacterial culture from the rectum was performed with negative end result. In wet smear of most Of the flower-horn fish free living organism was observed and in one case it was identified as protozoan parasite Balantidium.
Results & Conclusion: According to fish owners, beef heart and meat were the only food given to their flower-horn fish. Similar to all other cichlids, flower-horn fish is omnivores and requires a balanced diet including some plant material and algae. Lack of green food (as it causes constipation in humans) can over long periods, make the digestive track of the flower-horn fish more sensitive to bacterial or protozoal infections. Eventually, the "bloom" of micro-organisms in the bowel causes the anus to prolapse and get exposed. Any factor that increases the chance of bacterial or protozoal infections, such as poor water quality, can trigger the problem. An effective treatment should include the use of green foods only, such as cooked spinach, until the condition is improved. Fifty percent weekly water changes and the use of Epsom salt treatment were also recommended. The treatment proved to be successful. Finally we concluded that imbalance food and lack of adequate fiber to be the most important cause for anal prolapse in flower-horn fish.
IMO treating with Epsom Salt (Magnesium Sulfate) is the best course of action to take, as it helps reduce tissue swelling almost immediately. Regular salt (sodium Chloride NaCl) will not reduce swelling in the inflamed tissue, and can actually cause added stress on the fishes osmoregulatory system. The last thing your fish needs at this point is any additional stress. Withold ALL food for a few days to allow the gastrointestinal system to recover, and while doing that treat with metronidazole & praziquantel medication, such as Jungle Tank Buddies Parasite Clear.
http://www.junglelabs.com/pages/details.asp?item=TB636This will eliminate protozoa within the digestive tract of the fish, and will also assist in the fishes full recovery.
If the fish has never been dewormed then a 24 hr treatment of Levasimole is in order before one does anything else. (Magnesium Sulfate can be added while de-worming) Levamisole can be purchased in bulk under the trade name Vermisol.
If bacterial infection is suspected, treat with Seachem Kanaplex (active ingredient is Kanamycin) by mixing in fish food, and treat for 7-10 days.
Rectal prolapse that is not treated can lead to complications, which can ultimately lead to death of the fish.
HTH