Why does my fish have a swollen stomach?

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I have a six lined Wrasse in a 75 gallon tank that has a swollen stomach, the tank has 5 fish in total and 2 of which were bullies to my sick fish when my they first came to the tank but the bulling has lowered significantly to minimal possibly none. My sick fish is quite active and eats well, the only issue I know of is the swollen stomach. What is wrong with my fish and how do I fix it???