stress or playing

I've had a yellow tang for afew months now and he looks awesome and eats a ton but his behavior the last couple of ddaysdays have been different...he found my return line from the sump to the DT and swims straight into it then rides the current swims around some rock work and back at it all over again...just wondering if this is a sign of stress or he just "playing" around....tank is 90 gallons.