Thanks everyone -
BurnNSpy - my nitrate readings normally run between 4 to 6 mg/l NO3-N on my 55 tank, and slightly higher on the 30 gallon tank. The smaller tank reached 10 mg/l nitrate but water changes have reduced it back down. The small tank's only been set up about 3 months. I've been using the SeaTest Medium-Range Nitrate test kit.
The substates are not gravel - they are a mixture of aragonite and oolite sand. Much of the sand is below the larger aragonite surface.
Thanks for the reply.
Brian