You could also add to the sump, without lighting there would be no coralline algae growth, but the bacteria will be fine.
I have seen other people suggest putting LR
in the sump in stead of bioballs. I must object. What a silly waste of LR
. Put it in the main tank where it will be effective at reducing nitrate and enhance the fish's environment. Placing LR
in a high oxygen environment like a wet/dry would do much the same thing as bioballs. I don't get it.
I agree with everything else jackdp wrote. You need lots of LR
. Consider siphoning out the CC
and replacing it with aragonite sand.
The only other way to reduce nitrate in your current setup is with LARGE regular water changes to dilute the nitrate concentration. This is temporary and only marginally effective.