I doubt it's 7.2 for salt water. How are you measuring it.?Aragonite sand, live rock and skeletal mass will begin to dissove at >7.7 . with a bump in pH . 7.2 would indicate almost no basicity or alkalinity.
Crushed coral in a canister filter won't help. It wlli likey gather decaying matter and crate acidic conditions. As noted need some info on salinity, alkalinity a,calcium levels to be of any help. Aeration and fresh room air should help too.