Tetra Fat On One Side?

Question

To make a long story short. I just set up a tank a month ago and I haven't had a pet for over 5 years. We recently bought a tank. I have 3 fishes in the tank. I woke up one morning and realized that one of the fishes is fat on one side. It' looks like a bubble. It's a male tetra (Or so the lady at petsmart said) so I doubt its eggs. Any idea what it could be?

Thank You In Advance

Answer 1

give it a better diet