Does this CL look like it has skinny disease? It looked thin for a while, but I think thats because the other loaches bullied it until hiding, and it didn't eat anything. You can still see a small gap in the top of the tail fin.
I'd isolate it and treat it with levamisole hydrochloride just to be safe. Feed it plenty of meaty foods to fatten it up before it is too late. Once the skull becomes more pronounced, it may not be able to recover at all.