Help with nitrates

[makana]

I have been fighting high nitrates for far too long and don't know what do. My nitrates are usually between 10 and 15 when I test before a water change. I tried cooking half the rock first and then the second half. I have a shallow sand bed that cleaned up with a gravel vac. The tank is an all in one and I cleaned out the back chambers. Last week I did 25% water changes every other day and pushed the nitrates down to around 3 (tested on Sunday). Tonight they are 10 again.

I run filter floss that I change every other day. I have a skimmer that is rated for double the capacity of the tank. I don't normally run carbon but am right now. I have cheato in the tank and an turf scrubber in the back chamber (all attempts to lower nitrates). Nothing has worked.

The tank is a 28g nano cube with 30lbs live rock and a shallow sand bed.

I currently have 10lbs of rock in a cooking tub that came out of this tank and the nitrates in the tub are still climbing to 15 in a week, changing 100% of the water. They have been in there for 5 weeks now. Is it possible the nitrates are in the display tank are leaching from my rock?