Keep up KH and CA

[csaba911]

I have read many page in this subject but cannot figure it out why I cannot keep up with them.
Reef running for close to 7 years, in the last 4 month it's a constant battle to keep up my KH and CA, dosing 100ml 2x a day now from Bulk Reef supply's 3 part solution what I'm using for a long time now. Mag is keeping up 1280-1310, PH swings 7.9-8.2, using RO/DI water clean as it can be 001-000 ppm. Phosphate little bit high 5-30 "Hanna digital meter" depending if I feed heavy or not. I wonder if some other parameters chewing up my CA or 200ml of concentrated solution is acceptable. If I miss a day KH drops low as 6 and CA goes down to 350.
Tank doing fine others than a RTD wiped out a colony of acropoa a few months ago.

Any suggestions?
Ty