fish constantly opening and closing mouth
I added 6 banded barbs to my tank the other day, 5 of them are swimming around fine in a shoal, but the other one seems to be hiding on his own, is not feeding, and is constantly opening and closing his mouth. I don't think this is an oxygen issue as none of the other fish are exhibiting this behavior. I know that I have 0 ammonia 0 nitrite.
Any ideas on why he might be doing this?