Constipation occurs due to the lack of fiber in the food. Fish which are fed on only one type of food such as pellet based foods or worms may develop constipation.
Changing the diet to a more varied type can prevent constipation in fish. Aquatic plants are a good source of fibre. It is good to have live aquatic plants in the aquarium. Tinned peas are a good laxative for fish suffering from constipation. Epsom Salt (Magnesium Sulphate) can also be used as a laxative.