From a closer look, the holes are kind of symmetrical - the other side also have some similar-looking marks. These are not really holes IMO, but rather deep streaks, but I could be wrong.
Anyhow, just to be sure, I'm doing wc twice a week for a couple of months to see if the situation improved or not. Also, I added another lightweight canister filter (Eheim 2215) to the tank to help with the filtering - I only had Rena XP3. And I threw out the carbon bags too.