Plants remove nitrogen in all 3 of the forms we test for in aquariums.
Ammonia, nitrite, nitrate.
When the amount of nitrogen entering the tank is low, the plants sure can remove it all.
Fish food is an OK source in a low tech, high fish load tank, but with low stocking (8 xips in a 40 gallon, or most shrimp tanks) all the food that you are willing to add is not enough.
Which Flourish product are you using?
Seachem uses the name 'Flourish' for N, P, K, Trace, Comprehensive, Iron and Excel.