I've been running a dirt tank with a 3mm inert gravel cap for 3 years and I can tell you that you getting readable nitrates in your tank is very unlikely. Sounds weird? allow me to explain. My initial switch to dirt was due to algae problems. I had them all and in most cases nitrates were an ingredient or contributing factor and after a lot of stress and near giving up I set up a high tech dirt tank. For three years every nutrafin and api test kit I bought recorded 0 nitrates. And for 3 years no algae.
I have no scientific data to back up the following but I'm gonna say with a small amount of confidence that dirt has some type of binding effect when it comes to nitrate in particular. My stocking went up while using dirt and I once went two months without a water change and my nitrates remained at zero.
Make no mistake there were/are nitrates in the tank but my theory is they were bound to the soil because my plants grew relentlessly with no measurable nitrate in the water column.
Just my two cents...