I had one heck of a time keeping nitrates at detectable levels in my frag system (Salifert Test Kit). I was even dosing nitrates for about 6 months. I took a 3 prong approach and now they tend to be in the 2-3 range with regular water changes and I'm ok with that.
- Increase feeding. I used to feed 2 frozen cubes twice a week for a total of 4 cubes per week. Flake/pellets were fed when frozen wasn't. I started adding one additional cube of frozen 2x per week for a total of 6 cubes per week.
- Slowly added more fish. This tank is a frag tank and I had a few "worker" fish but the nitrates were always really low. I added two PJ cardinals and a small o. clown to the tank just for the poop. Right now this 60g frag tank has 6 fish that are all somewhat small.
- I pulled some of the live rock from my sump. When I changed the tank over, I took every bit of healthy rock and put it in the sump of the new system so I'd have a lot of bio filtration. Over the course of a few months, I removed a decent size piece every other week. I didn't want to go nuts and really throw things off so I did it very, very slowly.