could be deformed, could be over feeding, constipation, bacteria or parasites, egg impaction in females, gulping air, there are many reasons, are you in the USA or the UK? If you are in the USA then I would recommend Maracyn, if you are in the UK then I would suggest Myxazin, or any medication for fish that clears up parasites or bacteria, try the peas first and fast them for like two days, if it doesn't clear up then I would treat the tank. I usually give my fish peas at least once a month to help them poop and keep them regular.