Instead of CO2, add lots and lots of aeration, skimmer spray, venturi, lots of mixing, powerheads etc.
I dose a tiny amount of PO4, but mainly lots of Fe/Mn and KNO3.
Generally a decent supply will come from a main reef tank to a refugium, but not in these plant only tanks. Too much PO4= diatom blooms. So adding a pulse of say 0.2ppm or less 2x a week is about all most anyone would ever need to do from what I've seen with the species I've gown.
But a lot less is known about their horticulture in aquariums than plants.