Harliquin fish with giant syst? Please help! (w/pics)

[aabrey]

I have a small school of harlequin fish (6 in total) but realized when I get them home that one had a lather large whiteish colored syst on it's side. It's hasn't effected him really, he still swims around with the rest of them. The only difference I'm seeing is that he's not growing as fast as the other harlequin's and is now the smallest fish in the tank.
The syst actually bubbles sometimes and grows a second syst but every few days it pops and starts over again, a bit weird! It doesn't seem to be in pain at all but i'd like to see if there's any way I can help the little guy.

Below are some pictures I took, maybe someone can give me a solution?

 

[Apocalypse_Gold]

Welcome to AA! Sorry, we have no twelve-step program for treating cysts.
Try some heavy antibiotics (kanamycin, penicillin, tetracycline) or medicated food. QT if at all possible.

 

[Devilishturtles]

Agreed with A_G. Could be a simple tumor that won't go away. Could be in response to a bacterial infection. Dosing some antibiotics will tell. So long as he eats and swims well, I'd just keep an eye on him.