Shoving anything in there will restrict your waterflow. If this is an air powered sponge filter it will likely stop your filter from working completely.
It's driven by a 50 gph powerhead.
Buy some elodia aka armchairs, it eats through nitrates and is an easy grow and it grows floating
I'm gonna assume you mean elodea aka anacharish, and yes, I do have that in there, as well at watersprite and frogbit.
I would guess the sponge filter IS the problem. Its probably not enough filtration for your bioload and you would be better off with a HOB filter, even if its a small one.
All I have in the tank are 5 pygmy cories, 8 RCS, and 1 bamboo shrimp.
I suspect the cause of my high nitrates is from the osmocote+ I have placed in the substrate, perhaps I placed too much or not deep enough and it is leaching some stuff into the water.