A little story about the nitrogen removing ability of plants
Two weeks ago I had to go to France for a week and had my brother in law tend my tanks for me. I changed 50% of the water before I left, set up the autodoser and measured out the frozen food into daily portions and left for the week. I never turned the filters back on after changing the water in my 65 gallon.
My absentee keeper somehow didn't notice the filters weren't running, so they stayed off all week. The heater was off also, but my apartment thermostat was set to 74F, so no big deal. The lights came on for 10 hrs/day as they are on a seperate circuit.
When I returned that Friday, there was some GDA on my front glass, which immediately caught my attention as I haven't seen my little green film friend since last fall. I went to inspect the algae, and realized the filters were still off.
I turned the filters on and did a 75% water change after 5 minutes or so. The aerobic bacteria in the filter would have to be completely dead/dormant, and I had to immediately leave for NY state for another week so I did not have time to clean the gunk out of them.
I refilled the autodoser and left. I returned tonight after being away for another 9 days.
Other than the fact that the GDA is still on the glass, the tank is completely normal. Even the GDA is only a light film. There was no water movement, no filtration, no CO2 (luckily no BBA started growing) and continuous nitrogen input from fertilizer and food. There was no real recovery enacted, and no established filter media added to the tank. All there was to aid in water quality control was a dense plant biomass.
The fish and Amanos are all fine, and there doesn't seem to be any ammonia in the tank.
I'm most definitely not advocating running a planted tank without water flow or filtration, but this event seems to reinforce the conventional wisdom that filters in planted tanks perform a mostly mechanical role.