[tmz]

I doubt a phosphate deficiency is occurring in this tank with a 0.04ppm reading even though accuracy may be an issue. When a phosphate deficiency comes up a remover like gfo is usually in play.
Nutrient balance ( carbon/nitrogen/ phosphorus see Liebig's law) is important; the Redfield ratio (116 C to 16 N to 1 P)is an en masse measure of these nutrients in the sea and the ratio for may vary form organism to organism. Also as noted , nitrogen removal can occur via anaerobic bacterial activity which releases N to form N2 gas which bubbles out of the tank which can skew the ratio in a given tank toward lower N.

In theory organic carbon can be lower than desired limiting the growth of heterotrophic bacteria which can't get it from CO2 as photosynthetic organisms can. Growing these bacteria is the reason for dosing organic carbon( vodka ,vinegar et al).As they grow and multiply they take up P and N along with the increased available organic carbon and they and their waste products are exportable via skimming. Sometimes nitrogen deficiencies can occur when carbon dosing ,rarely if ever a PO4 deficiency though. None , in my 8 plus year experience with carbon dosing anyway.