Discus: Breathing hard on 1 gill

I noticed that my recently purchased Discus is breathing heavy on 1 gill
about 2 gulps per second.
There is no flicking on objects and there is no darkening of body. He
doesn;t hide
in 1 corner and has a normal appetite. The other fishes (Cardinal Tetras,
Emperor Tetras, Ram, and Colisa Gourami) in the 65 gallon tank looks OK. The
other Discus is doing OK as well. I an guessing this is not so much ammonia
or nitrite since this will cause the other fishes to be stressed as well.
And the test kit says the levels are OK. I am also using a DIY CO2 reactor.
I notice some of the mix get pumped into the tank, miniscule amt but enough
to make a gooey stuff around the CO2 outlet before going to Dennerle style
mini ramp diffuser.
Other than that, there is nothing else in the tank that will make my Leopard
Snakeskin Discus breathes so hard on 1 gill. It's been in the tank for 1
week now. Could it still be gill fluke? Any suggestion for quick remedies?