This morning I noticed one of the male barbs just sitting in the corner for an extended period. I got suspicious and looked closely at him. I noticed he has a pimple-like white spot right in the centre of his snout, slightly raised (not just the white part, but the skin around it, like a pimple on human skin). There are no more spots on the rest of his body. I don't see anything similar on the rest of the fish. What is it? He sat in that corner for a couple of hours, then suddenly got out and joined the school as normal. Nitrates - 10 ppm, nitrites - 0, I didn't test ammonia because honestly I can't see how there could be any (the tank is cycled, no recent changes).