John,
The reason your
Nitrates are up a bit is because a
DSB, activated with
aerobic bacteria from LS, is not necessarily activated with
anaerobic bacteria in sufficient numbers to convert
Nitrates to Nitrogen gas. That action happens in the deepest layers of the
DSB. Two things need to happen BEFORE any conversion takes place. First
anaerobic bacteria populations must multiply to sufficient numbers to convert the available
Nitrate. Second the
Nitrate available has to reach the deep layers of the
DSB. Natural water circulation happens in the upper layers of the sand bed assisted by your sand stirrers. Since there is no natural water circulation in the lower reaches of the
DSB the
Nitrate laden water has to be transported there by another means. Below is pictured my 75 gal reef as it was being set up in 2002.
It's easy to see the 5"
DSB has two layers. The deepest is 4" of
Aragonite sand and the upper layer is 1" of LS. This initial "inoculation" of LS is just the beginning. If that is all you add to your
DSB it will be awhile before it can convert all the
Nitrate produced as you add critters. An essential addition, one that many reefers new and old neglect, is LS bed activators. The most important of which is "spaghetti" worms.
Spaghetti worms can be gotten on-line or from a reefer friend's established aquarium that has them. In the above picture it's plain to see the worm has tunneled into the sand bed. When they move through their tunnels they push water ahead of them, pulling water in from behind. It's this water movement that takes water to the deep layers of the
DSB where
anaerobic bacteria perform their magic converting
Nitrate to Nitrogen gas.
This picture shows the same aquarium about two years after setting it up. The
DSB is healthy, functioning properly keeping the
Nitrate below 10
ppm and usually 0
ppm. It's beneficial for some critters, like clams, to have a small amount of
Nitrate in the aquarium. Nitrogen gas can be seen in the sand bed on the right below the Fungia coral.
I hope this helped clear up the reason for the
Nitrate reading you are experiencing.
Dick