Guys. It is of my opinion you are reading ( or not specifically actually
) too much into this dosing of Nitrates, in liquid form, phenomena.
The best/least complicated, and likely most enjoyable, way to supplement NO3s, in order to more efficiently lower PO4 in a system highly capable of processing these extra nutrients, is to elevate the biological load via increasing the number/density of fishes ...
*In a responsible, knowledgable fashion ! I cannot stress this enough !
While Im no longer intimately knowledgable on the specific details of Freshwater Planted products, I would imagine Seachems' Flourish additive is likely to contain elevated levels of Fe along with NO3. While the lack of knowledge on the original topic is apparent, despite some thoughts/comments assuming otherwise, Id imagine I dont need to go into specifics on the negative aspects of overdosing Fe and/or NO3 ...
More relative/useful for the "ADVANCED" practice of dosing NO3s to lower PO4s would be a concentrated, highly pure form of KNO3 ... Especially in a bacterioplankton driven system where unsupplemented K values can/will get depleted to the point their surpressed levels become detrimental to the more sensitive sleractinian organisms, specifically species in the genus Acropora.
*Again, I cannot stress enough the importance of FULLY understanding/contemplating these advanced topics before their application !
This topic, as a whole, isnt one youre going to be able to successfully grasp and implement hastily, especially through the use of "wikipedia" or any other shallow, unprofound source(s) of information on these subject(s)