Tank PH issues

[Focustronic_Jonas]

My tank is a Nuvo 20 (13g of water) heavily dominated by SPS

Alk : 11dkh ( accidentally raised it beyond and trying to get it back in control slowly. Usually 8dKH)
Cal : 420ppm
Mg : 1400 ppm
No3 : 7-10ppm ( this is from dosing Neo nitrate since reading was 0 ppm few weeks back)
PO4 : 0.1-0.2 ppm (fluctuates based on feeding)

Dosing is All for reef : 23ml
DIY Nopox : 1.5ml

My ph used to swing between 7.8-7.9 over night to 8.2-8.3 day time.
If it ever drifted lower ny Alk was lower than 8 and I used to correct it using BRS alk solution to bring it back.

This has been stable levels for last 4-5 months.

I recently switched to Aquaforest Reef Salt from Red Sea blue bucket because the Red Sea always used to have lower reading requiring correction plus storage issues and parameter stability reading re-correction post waterchange.

The aquaforest has been spot on for cal and mg and alk bit lower than 8 but easy to correct.

However my ph trend has changed. Now I bottom out at 7.5-7.6 with 8.1-8.15 as day highs and today it maxed out at 8.05.

My high KH is from accidentally dosing too much Seachem Balance using apex based on Ph to try and stabilize the overnight ph.

I have left windows open, etc to try to see if it help with co2 but no change.

What could be going on? I suspect something with the salt mix because that’s the only change in system but what could be in the salt to cause this?

I did a 5g waterchange yesterday and my daytime high was 8.2 and night time 7.8 but something changed as day went on.

Should I swap to a different salt, if so what?
What options do I have to correct it.

The corals are still ok and growing.

As long as alkalinity is same I have hard to believe its the salt, that should in that case be reflected in lowering the alk. First of all I would recalibrate the electrode, as it's a very common reason of bad readings. One thought though, some salts contains carbon (I dont like that), if thats the case that could be a reason as that could maybe increase bacterial growth if you were carbon limited, and in that case lower pH.