PH Problem

I checked the calibration using the calibration fluid from Hanna.

PH: 7.9
DKH: 13.1
ALK: 4.69 meg/l
Calcium: 300 ppm
Magnesium: 975

Tank has been up for approximately 5 month, Tank only has live rock with deep sand bed and yellow tang, clown fish, sea erchant, ceirth snails.

All the fish are doing fine. I am tring to get this tank ready for reef, as soon as I can cure some more live rock, and get the water right.

One other thing PH of water used for water changes is 9.6.
I would add some calcium, and let alkalinity drift down on its own to no more than 4.0 meq/L. Ca is very low. These articles should be useful:

Some B-Ionic might help with calcium and alkalinity, if the tank isn't too big. That's a good way to get started, anyway, IMO.

The magnesium should be around 1275 ppm. Kent, ESV, and Salifert make good magnesium supplements, and there's a DIY formula around as well.

Back to the pH. More aeration might help. Also, I think a second opinion on the reading might be useful. Meters have been wrong in the past. What brand of calibration solutions are you using, and how long ago was the unit calibrated?
Calibration Fluid from Hanna. Two step process with fluid PH 7.01 and fluid PH 10.01. Did it tonight because I thought the reading was to low. Fluid and tester are brand new. Just got them. I will check PH again using Salifert test. Will post shortly.
I'd guess that it's a CO2 problem, then. You could try getting your parameters in line with perhaps some calcium chloride product like Turbo Calcium. Then you could look into a lime drip.

Does the tank have a protein skimmer?
I have a hang on skimmmer for temp. Looking at a Aqua C ev2000 with the Aqua C Auto waste Container.

When I set the tank up originally, LFS told me that I didn't need a skimmer. Bought the hang on skimmer for temp. I know this skimmer is not large enough for my tank.
Well, more aeration might help, and a bigger skimmer would definitely provide more aeration. A lime drip might also be needed, though, if the CO2 level in the building is too high.