I used to have pretty frequent fish gasping, especially after large WCs. At one time, I suspected my Excel as well (I was thinking it might have expired or some such). I recently added an air mister which turns on when my CO2 turns off. Since then, I've not noticed a single problem. So, in my case, I think I was typically running at lower than desired O2 levels.
So, in my case, adding O2 injection seems to have helped.