I have a black widow tetra that since last night have been constantly opening and closing its mouth really quicky, I thought at first it might have something stuck in its mouth but it doesn't. It isnt gasping for air as its swimming nowhere near the top, all my other fish are fine and I change my water every week.
It seems to be swimming okay, maybe slightly to one side but I can't understand why its constantly moving its mouth

Can anyone help me?
Thank you