11-22-2012, 03:58 PM
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If this happened over night, chances are it isn't from a parasite or anything, but it's possible I suppose. It looks like he ran into something sharp or got a nasty bite from someone. Normally bites take off the top chunk, even make a long rip over leaving a round hole like that though. It may even be some sort of cyst that blew(not all cysts show up as a bump if they're deep, and when they blow they can leave a nasty crater), it's deep, round and looks a bit yucky, so a cyst or running into a sharp object is my best guess for an over-night ouch like this.
I also agree you should get a bit of medication for him, something safe for him and all your fish that is a good antibacterial. You may even want to get something to kick fungus around too, with a nasty like that it's better to be careful. Make sure your water stays nice and clean too. =) You may actually want to move him to a QT tank to treat him if the meds that would be good for him aren't good for your other fish.