Sick fish in quarantine.

sounds like ich. I would say try upping the temps in the tank and give him a day or two to see if it can fight it. If you're really worried about its health or it looks pretty bad you could start an ich treatment in the tank now. Unless you plan to quarantine inverts in the tank later I wouldn't worry about the type of medication. If you do plan to quarentine inverts you might want to try copper free medications that are invert friendly. That's why I'd try raising the tank temps first to give them an edge to fight it. Fish can get over ich themselves if they're not too stressed and the water is warmer than usual. A water change wouldn't hurt them either to keep water conditions less stressful.

If it is ich it affects your whole tank. Ich breeds in the substrate, so if one fish has it most likely all the fish may get it. I would find a treatment that is good for the whole tank including inverts. I have used Mardel's Coppersafe, it worked very well. Also I have heard that Kent's RX-P works great, it's made from natural plant extracts and is safe for most anything in the tank.