It looks like an injury and since you are saying that the swordtail was nibbling the fins, he has had some other hostility of some kind from somewhere. *I do not see any indication of what other fish are in the tank but he needs to be quarantined to heal and he is going to need a med to heal (NOT Melafix or Pimafix) Maracyn 2 would be good for this type of injury. *(or Furan 2) *It could be that one of the other fish has tried to take chunks out of him. *Do you by any chance have any other gouramis or, heaven forbid, a betta in there with him? *If so, you have a problem. *Two fish of the gourami persuasion can fight to the death if they feel crowded, or if it is the first Tuesday of the month *

, or if they have a tummyache, or whatever reason they feel makes a good one, sometimes no reason at tall. * (BETTAS are of the gourami family and we have had deaths of both gouramis and bettas in tanks where they were placed together)
Since you have him isolated now, give your tank a good looking at and see that there is no decor that has sharp or pointed edges of any kind, plants that have any types of protrusions that may have poked him. *Also could you please let us know what other fish you do have in your tank.
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Happy New Year
Rose
