N,P,K - Nitrogen, Phosphorus, and Potassium, the 3 main elements required for plant growth. If your tank is limited or bottoming out in either N, P, or K, it can cause algae growth. Also, if there is too much of either N or P, it can cause algae growth, and also harm your fish. K is not really dangerous even in great concentrations, so there is really no need to test for K. I doubt it is the problem in your tank because it is not really associated with algae growth (unless you have 0, but since you are dosing it, we know you shouldn't be bottoming out on K). You test for N by testing for Nitrates... a standard aquarium test kit. You test for P by testing for phosphates, PO4. You can find a PO4 test kit at most aquarium stores, and definitely at any stores that sell salt water/reef stuff.
Fe - Iron. One of the many trace elements required for healthy plants. You can test for chelated iron by buying a test kit for it at most decent aquarium stores (probably not petsmart or petco), but it is a pricier test kit, and really not required. We know you are dosing it along with K in your tetra florapride. There are more trace elements that planted tanks benefit from, but dosing them is probably not required in your tank unless you move into higher light and CO2 injection.
