Wet/Dry filter in small tank and high nitrate problem


New Member
Aug 20, 2017
Cracow, Poland
From half year I have running the small 16 gal tank with built-in wet/dry filter (The link below).
The filter works perfect (any algae problem, any mess in the water, very guick cycling at the start, good O2 level etc.). However I have with nitrates >30 ppm. I try to dose no NO3 but NO3 still remained relatively high about 20ppm even a 70% water changes (per week).
The 95 % of plants do very good but some not-high-NO3-lover with mixing succes:
In the filter I have: 3 litres of plastic biobals, huge flow about 2000 l/s. I have also 700 ml of Seahem matrix and 500 ml lava rock in the submersed part of the tank.
Above the wet dry part I have two sponges to mechanical filtration (one coarse and very fine = about 2 L)

My quaestions to wet/dry user are:

1. Will I should reduce the amount of biobals, or increase the water level to reduce aerobic conditions for bacteria ?
2. Or ad some more material for sumberesed part of the filtrer to give the place for deniitryfying bacteria (e.g. more Seahem matrix)

The last options about i think is to completly remove the biobals, increase the water level in the filter and add some sponge to the previos wet dry section. Doing this i will still have some extra oxygen in the system but less nitryf ication.
Thank in advane