Piranha

Question
My grandaughter has a large tank with two large piranha; both about four years old.  Yesterday one was found upside down.  Whenever she flips it over, it turns upside down again.  It seems to be lively and still eating.  It also seems to have cloudy eyes.  What might be the problem?

Answer
How big of a tank are these two pirahnas in?  They need to be in a 200+ gallon tank with at least 3-5 in a group.  how often does she change the water?  With pirahna, they need their water changed 50% 2-3 times a week as they are very messy eaters.  It sounds like a swim bladder problem along with cloudy eye. This is usually caused by poor water conditions and stress.