Tank probably wasn't ready for corals of any kind. You're going to be stuck dosing to maintain it.
There are three sources of coral food in a mature tank -
1) Macro Algae die off. As some grows in your refugium, some also dies. Ironically the dreaded GHA is pretty great at this as long as you keep is confined to the refugium.
2) Microfauna - both directly, and through their waste & corpses. Copepods, worms, brittle stars, etc.
3) Fish poop. Can't be overstated. Fish poop is far more effective a coral food than fish food is.
You don't have any of these. If the tank is old enough to have all three then measuring 0 phosphate and 0 nitrate is no big deal; there's still plenty of it around it just is being used as quickly as it's released.