38 Gallon Mixed Reef Cube Newbie

[Tigweldpro]

Hi, quick update and a few questions.

My tank has been running for 6 months now and I'm having an issue with coral health.

Temp 77*F
Sal 1.025
PH 7.9-8.1
Alk 8-8.2
Cal 425
Mag 1300-1325
Nitrate 15-20ppm
Phos >>>0<<<

I am doing weekly 5 gallon water changes(reef crystals)
Dosing 1000ml Kalkwasser @ 50% strength with dosing pump at night(61ml/hr)

My Candycane, Frogspawn, War coral, 2 RBTA are all not doing well. Surviving but not growing or looking good. I have been testing and keeping my water very steady but I have tested phosphate with 3 different kits and it is dead 0 like not even a hint of color at all.(API,Salifert,Red Sea)

Is it possible that the Kalk is precipitating out ALL the phosphates and my oversized bubble magus curve 5 skimmer is removing too much?
There is almost no Algae in the tank and Cheato will not grow in sump(slowly dying)