With an external prolapse it's the wall of the fishes ass hanging out, not his intestine. That's why it's called a
rectal prolapse.
Water quality is not the cause.
In humans these are some of the more common causes:
•Malnutrition.
•Deformities or physical development problems.
•Straining during bowel movements.
•Infections.
Also in humans, often times surgery is the only corrective action that can be taken. Which is also why Tom and I both stated sometimes they retract back into the body, and sometimes they don't. The longer this goes on, the less chance of a full recovery.
http://www.fascrs.org/patients/conditions/rectal_prolapse/