Sorry To Be A Pain But My Sailfin Mail Needs Help And Quick

HELP!my male sailfin molly has like a big gash well it looks like a hole but with shredded skin its horrible poor thing must be in pain please help.ive never see this before i have a zebra lauch that thinks hes the king of the tank hes about 4-5inch long and seen him a few times go for my fish i have 2 of them should i sell them as i cant see this again this has realy upset me the spot is at the top around the centre

well i dont know were the smaller zebra lauch is so its just the big one

How the fish doing.
I would issolate the poorly fish and add a bacterial med.
How many gallons is the tank.
How many fish and which type.