Betta Fish: Sushi heavy breathing

I just moved Sushi to a 15 gallon tank from a 6.7 gallon. Transferred water, substrate and everything else from old tank to new tank. Water parameters are great, heater yes, filter yes. I added 3 Cory’s and he is totally fine with them. What worries me is this breathing that just started today.
He is swimming fine 99% of the time, but every once in a while, I find him like this. What could be going on??? Water filter was strong- which definitely irritated him- I fixed that issue.