HELP - High pH in marine tank

I am experiencing problems of high ph in my new reef tank. I have kept reef aquariums for 5 years now and this is a new set up. It has now been running for 2 months.

First here are my tank specs
800 litre display tank
300 litre sump
2 jeboa power heads (can't remember the exact spec but I run them on 50% and get x20 turnover)
TMC 1500litre skimmer
Running bio pellets in reactor
Running carbon bags in high flow area of Sump
2 radion and 1 max spect razor over tank
1 maxspect over sump
90kg of dry marine rock, this rock was brought dry and no live marine rock has gone into the tank
For seeding the tank, I used the Red Sea acclimatisation kit

I use a seneye reef to record my ph and I also cross test with a drop test kit.

My ph runs are 8.6 - 8.7 over the day night cycle.

My ph has become a steady up down cycle in the last 20 days.

I have tried adding distilled vinegar. Which brought my ph down to 8.3 but it went strategy back up again.

I also run air bubbles through the sump. I have two large twin air pumps. 1 runs two air stones and the other runs to bubble filters. There is also a waterfall built into my tank, so I don't think lack of carbon dioxide is too much.

So far I have some frags (zoos,lps and Sps) in sump and some crabs and snails in the main tank.
The coral seems fine and although was only fragged 1 month ago, most pieces are doing fine.

Please help, as I don't know if the ph is fine, or I should start dosing to keep it down?