Unless your carbon dosing your not nitrogen deficient so don't worry about raising No3
As for gfo any phosphate removing media works , chances are yours is exhausted as it can only hold so much phosphate . The first batch is used in 48hrs if it's high Po4 then each batch afterwards will last longer as long as your inputs don't over weigh the exports
High phosphates inhibit growth in hard corals
A high alkalinity would actually get you better growth in hard corals coupled with a higher ph , but yours is a tad high leaves no wiggle room.
Lastly buy good test kits , your Po4 may not even be accurate if it's not verified
