No worries bud. superquick 'very general' overview
Nutrients = wastes
Wastes need to be processed.
NH3 (ammonia) , & NO2 (Nitrite) are converted via Oxygen loving bacteria.
(NH3 is also assimulated via plants in various settings.)
NO3 (Nitrate) is converted via Anaerobic (Oxygen Limited) bacteria, assimulated by plants & diluted via waterchanges.
Looking at your pic, shallow gravel base, high O2 priority, few plants, & mechanical filtration. This rules out anaerobic conditions in the bed, possibly the rock, the filter, leaving only the plants & water changes to have a crack at your trates.
Thats the natral route at least, im sure plenty love their chemical approaches.
Just asking the question at any rate. Food for thought