PH refuses to go up!!

[Hightower]

Ok been working to get the PH up for months now. Nothing has worked. Im at a deadend it seems and the xenia is taking it personally. PH refuses to go above 8.1 avg is 8.0 and goes to 7.9 at night.

I drip/topoff with kalk. Airpump running fresh air through airstones in sump to drive off excess carbon dioxide.

Alkalinity is at 10DKH
Calcium 400
Magnesium 1300
Nitrites/ammonia 0
Nitrates sit at 5 right now. usually 2 or less. (Water change coming up)

90 gal system with lots of flow, with a 10gal refug with cheato, and a 30gal sump with an ASM G2 that pulls a decent amount out.

The only time the PH goes up is when I add an alkalinity additive. Usually randly holmes farley recipe. Thats to be expected. But it doesnt stay up.

What else could it be????