Leave a dirty sock in for a few extra days. Guaranteed to increase nitrates.
Also, 0 nitrates isn't necessarily a bad thing. As long as you have biomass to uptake the excess nitrates.
If you don't want to dose nitrates(no idea why not? Neonitro from brightwell is an easy way to start), adding more fish, or just feeding more and allowing it to rot creating ammonia is a good way(more ammonia, more nitrates after it's broken down).
Also, 0 nitrates isn't necessarily a bad thing. As long as you have biomass to uptake the excess nitrates.
If you don't want to dose nitrates(no idea why not? Neonitro from brightwell is an easy way to start), adding more fish, or just feeding more and allowing it to rot creating ammonia is a good way(more ammonia, more nitrates after it's broken down).