Is there a difference in the phosphate when it is in solution if it came from organic rather than inorganic compounds?
Actually, in a broad sense, yes, there is a difference, but that difference is NOT what The Matrix would have you believe.
The difference lies in the origin and the location of PO4 within large organic structure. The PO4 ion itself is the same as inorganic PO4 but in organic compounds it is attached to other molecules and is not free. As it is bound to other molecules it's activity is neutralized until the compound is broken apart by some physical or enzymatic action. Inorganic PO4 is free as soon as it is dumped into the tank and is immediately taken up by plants as well as by bacteria. On the other hand, PO4 attached to organic compounds still counts as an organic compound and is ignored by plants, but the compound itself can be broken apart by bacterial action. None of this is actually relevant because just about all organic compounds are constructed of some combination of Carbon, Nitrogen, Phosphorus. Potassium and other elements. These elements are bound up within the architecture of the complex organic molecules.
So getting rid of organic compounds (regardless of whether they contain PO4 or NO3 or whatever) by changing the tank water is important because the breakdown of organic compounds in the tank by aerobic bacteria consumes oxygen. The Matrix programs it's little lemmings to get rid of organically derived PO4 in the belief that PO4 is a scary boogieman, but the danger lies in the actual breakdown of organic compounds - full stop. The more complex the compound the more oxygen gets robbed from the water column. It is therefore pointless to worry about organically derived PO4 because by the time PO4 is released from the original organic compound the damage has already been done.
This is exactly the same for those that fret endlessly about NO3 in the tank. Again, they are programmed to fret about NO3 and so panic sets in if their cheesy test kit shows high NO3, but the NO3 is the result of the detoxification cycle NH3-->NO2-->NO3.
By the time the innocuous NO3 is constructed and liberated by bacterial action the damage has been done by the toxicity of NH3 and of NO2, plus the removal of oxygen from the water column by the bacteria to get to the NO3 end product. Frequent and large water changes defends the tank against the damage by reducing the concentration of NH3 and therefore reducing not only the exposure to the fish, but also reduces the need for the bacteria to steal oxygen from the water in order for them to
produce NO3 in the first place.
At the end of the day therefore, it is pointless to be concerned about organically derived NO3 or PO4. In fact, if these ions are liberated from their organic construct then that is probably the only good news of this melodrama. The bad news is that fish and plants are at risk of exposure to highly toxic NH3 and NO2 as well as to hypoxia, which is the worst possible news.
Cheers,