the bottle i tested was 'ready to use' according to the seller. So not every source of fyto may have 0 nutrients in the water collumn.I currently culture 6 types of Phyto and this is what I have learned.
When you start a culture, you add the fert. as food for the phyto along with light. As the Phyto grows and multiplies, the fert is consumed by the phyto. Even with thalas, since you have to add silica for them to grow, they utilize that to grow and multiply. when the culture is ready, there will not be any real detectable amounts (if any at all) in the water since the phyto has used it thus, it will not go into your tank. Remember that Phyto is essentially a plant so, the main waste product of phyto will be O2, oxygen.
Also, any Nitrate or phos that is present in the phyto water, will be consumed by the phyto at some level.
But that the algae consumes it doesn't make it disappear. It either gets eaten and then pollutes the one that feeds on it with the trace elements and other waste. Say you feed the fyto and it gets eaten by zooplankton (i.e. copepods) then you get a process called biomagnification if you have say a mandarin dragonet that then eats the copepods, the levels of copper or whatever pollutant was absorbed in your phyto has now increased lets say 50x.
also a disclaimer that this is all in theory and I have no idea if this would go this way in a reef tank but it certainly does in nature