Feeding the fish results in increased N or P, but rarely (if ever) both. In a planted tank the increased N maybe of little concern, but if there's excess P (by itself or in combination with the N), a lot of CO2, and a lot of light then we must know how the food that we use affects the N and P.
On some Dutch sites one can find a table that shows how different foods increase N or P. That may seem completely useless to some because here in the US we like to dump spoons of dry chemicals in our tanks, change oceans of water twice a week, and generally believe that we know exactly how to calculate fertilizers we provide to the plants. The Dutch use a variety of very natural approaches to maintaining a planted tank and battling algae. The fish waste is a variable that seems important to them. Strange people, I tell you...
I'm away from home now and I will have to post the Dutch food fish table in a few days if someone doesn't do it before me.
From what I've observed (but not tested precisely) dry foods tend to raise P a lot and not increase N much. I do not know if there's a difference if the food stays uneaten or it "passes through" the fish. Another good question is if the amount of N and P in the fish waste depends on the species of fish.
As a general "good housekeeping" practice I personally try to never let any food fall on the bottom and stay uneaten. I just feed carefully with a pipette, but one can assure that no food remains uneaten by having different species of fish and/or different sizes of fish.