Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think you are right that at the light and CO2 levels you are currently running the plants are not removing all the ammonia/nitrite/nitrate.
Even when you add more fish this may not be a problem, though, as long as you are willing to do water changes.
Or you could go back to higher light and more CO2.
The other way is to grow plants that are above the tank, in the air, with just roots in the tank. There is a lot more CO2 in the air, and if the tank is near a window there is enough light for most house plants.
I have several plants in various tanks and the N readings stay quite low.
Epipremnum, Syngonium and Philodendron have all done well for me in this sort of setting.