There's not much you can do if your fish isn't eating. Maybe he's just not having a good day. I used to take care of a fish who wouldn't eat for a day or two, and then eat everything in sight the next day. How long has this been going on? What's your water parameters (ammonia, pH, nitrate, nitrite, etc.)? Is he the only jack in the tank?

It's not likely that he has a parasite, if he did, he would be eating a lot.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."