I have a yellow tang that was doing well in my 300 gallon tank. I switched off my protein skimmer this morning as I normally do after a partial water change as there can be alot of small bubbles. Came home after work today and the yellow tang is struggling, bit like he didn't have enough oxygen ! All the other fish are ok , checked the water parameters and all ok . He has disappeared into the rocks now which is unusual for him and not feeding. Any suggestion?