about a month ago i wanted to add some tiger barbs to my community tank and i picked some up. all but two died within a couple days, which i thought was solely my fault until i went back to the pet store a few days after purchasing them and the tank of tigers were not for sale anymore because they were all sick. none of my new fish had any visible signs of disease though.

luckily, the barbs have not been nipping at any other fish like i've read they would do in groups of under 6. i have neon tetras, a dwarf gourami and a few albino corys in the tank with them. (a 20 gal long with one melon sword and bunch of plastic plants)

in the past few days i've noticed that one of the tiger barbs has a weird problem with his mouth. it looks like he has a fat lip that extends out on the sides and hangs down a little. i haven't been able to take a pic yet because he's still active in the tank and i can't get a good clear pic of the mouth.

when i feed them, he stays near the bottom and won't even recognize the food, even if i put it right in front of him. i feed them hikari tropical flake food, (and hikari wafers for the corys) and have been for quite a while, so they are all used to it. he is able to swim at all levels of the tank, but he won't go up for food lately.

ammonia and nitrites are at 0, and nitrate is at 5. i vacuum the rocks at least once a week and change 20-30% of the water. none of the other fish have any symptoms of sickness.
i can't figure out what the disease my barb has, (or if he even has one), so i'm not sure how to treat him.
please let me know your advice if you know anything about this! thanks!

Hard to tell what disease... but it is certainly sick! T-Barbs are voracious eaters so if yours is hidding and not eating than something is definitively wrong with it. Transfer the fish to a hospital tank (a smaller tank with adequate filtration where sick fish are isolated and treated) to avoid spreading of the disease.

Is the lip covered with white cotton-like material?

i transferred him a few days ago to a hospital tank and decided it looked most like mouth rot...or something along those lines. i treated him with some maracyn plus but he ended up dying yesterday. there wasn't any cotton-like material, so that's why i didn't know what it was. it was the same orange color that the rest of his body was, it just hung down lower on the sides of his mouth. now i only have one left! i've been watching him, and he isn't attacking any other fish (i think the dwarf gourami keeps everyone in line!) but i'm on the lookout to find him some new friends so he's not alone. from a different pet store probably!