Well my only other guess is a heater burn, and if you don't have a heater then its probably ammonia burn, especially since he isn't eating. small water changes daily, some seachem stability and seachem prime to help mitigate the toxins in the water, and go from there. 10-25% water changes, every day along with those 2 products I mentioned will get your tank under control. You didn't let the tank cycle, and adding all those fish at once surely caused an ammonia spike, especially ones that eat as much as they do. If your water test comes back with no ammonia or nitrite, then he banged his head off something. Add salt to the tank to mitigate bacterial/fungal secondary infections and let him heal up.