if his mouth has always looked that way, then i wouldnt worry. something that happened to him before you got him. can he eat ok? is he acting otherwise normally? if so dont worry, keep up with his waterchanges and all is well.
*to all other posters*
there is something amiss on this sub-forum since loads of "old reliables" left. it doesnt bode well to downtalk the poster when things go wrong or say well you should have... main thing is they are concerned NOW!!! be glad they are asking for help and help we should give. find a solution to current problems and NOT criticize for current situations(weve ALL gotten bad advice/guidance). i feel so sorry for "noobs" here really showing that they actually CARE about their fish. some are young and not a lot of money. so solutions have to be found within parameters mentioned.
with that said i hope we treat folks a litter kinder around here and soon. im almost fed up with the childish games. itd be a shame to leave with the knowledge/education i have and sorry if it sounds as tho im tooting a personal horn. but you GOTTA give people credit for at least showing concern and asking for help. a bigger tank or a stronger filter isnt always the easy solution for some poeple.
cheers everyone. not trying to cause a disturbance or raise some hairs but courtesy and common sense are lacking around here.
to the OP, let us know how you get on.
all the best