I use Red Sea Coral Pro salt and use Red Sea Pro Multi Test kit for Mg and Ca and a Hannah Alk Checker. I use API kts for NH4, PO4 and NO3.
Lighting schedule is Kessils on at 7am off at 10pm. They ramp slowly to 30%/30% peaking at about 2pm. Actinics come on at 9am and off at 9pm, coral+/blue+ come on at 11am and off at 7pm.
Ditch the API kits. Get an ULR Phosphorous Hanna checker and one of the brand name marine nitrate kits. I use the Red Sea Pro. If you're running GFO pull it. Stop dosing 2 part. Do a series of 25% water changes every couple of days until things start improving. I'm positive you're causing alk and calcium to precipitate out. I'd pull the carbon too. Don't just shut off the reactor or you'll create a horrible sulfur pocket when all the life inside dies and decays.
My 57 only requires 1 tbsp of kalk in 5g to maintain levels.