Usually, fish yawning occurs as a display of territoriality or to attract females. If your fish is yawning at nothing, it's for the same reason people yawn...an abundance of CO2. If your fish are yawning all the time, do a water change and boost aeration in the tank.
I wasn't talking about fish. I was referring to your assumption that people yawn due to an abundance of CO2. That is just one hypothesis, and apparently there is no actual data to support it.
*HUMPH* did gravitate not ask "DO FISH YAWN" ? well this is the reason i posted that site you are the only person who's talking about humans yawning, no one else !