About to throw in the white towel

[Coleus]

Well i am battling this ph problem that i have with my tank for couple months now and about to give up. Here is when it start

Since i used the bio pellets my ph has been between 7.7 to 7.9 which was 8.1 to 8.3 before. I used RO/DI water with TDS = 0 for my water change and ATO.

My current calcium level has been stable at 420-430 and my alk is 11-12. my mg is also around 1350. I currently do two part dosing so all those has been stable. My testing tools are 3 ph probe, two different test kits Salifert and EOS. My nitrate is around 5ppm, not sure about phosphate because i don't have any algae problem

I know as long my alk is high i should not worry about my ph is being low. But i have been tried to raise my ph with 3 methods

1) PH Buffer from seachem, I put in the amount to raise it to 8.1 at night (as my ph probe read 1 hour late) and in the morning, my ph went back to 7.7 :-(

Doing this also raise my alk a little bit like to 13

2) Lime water (using kalk): Put some in and raise it to 8.2 and then in the morning, wake up and it is still 7.7. Doing this won't raise my alk or lower my calcium much

3) Put in some air stone, still nothings. I tried to open windows during these hot days but still nothings


What else i can do to raise my ph? This is very frustrating. I also have a refugium with chaeto, mangroves. Although chaeto has been shrinked because of bio-pellets


Any inputs is greatly appreciated!