I forgot to ask, but do you have any genesis blocks in the sump? If not, pick up their 4 pack with holder and put it in a low flow area of the sump. It will slowly bring down NO3.
Sorry, I don't but it might be too slow? I have a lot of rocks inside the DT already but with 30+ fish and heavy feeding, nitrate is not budging. My other 2 smaller tanks are carbon dosing with much success (NO3 < 15ppm and steady) but I have yet to get this tank down to 10ppm. I think that my problem is PO4 limited. Carbon Dosing will feed bacteria to break down NO3 & PO4. However, mine kept stalling since PO4 is depleted first; so I kept dosing NeoPhos to compensate. I'm currently dosing 50mL/day of NeoPhos -- ugh!
Anyway, I've just ordered 12lbs of sulfur from Geo's Reef (I only need 2lbs though) so will restart my sulfur reactor soon. It dropped NO3 from 100ppm+ before so I'm confident it will work again. Once dialed in -- it's super simple to control & maintain.
The downsides of sulfur denitrators?
1. Can release hydrogen sulfide (H2S) - rotten egg smell if no flow at ~ -300mV. Effluent will be controlled via a Kamoer and I have an ORP probe inside the reactor. I've found the sweet spot to be between -100mV and -200mV.
2. Takes 3-4 weeks for bacteria to colonize inside the reactor
3. Consumes 5-10% alk. It's easy to counter -- just add a little more soda ash. My alk is ~8.3dKH, dosing 75mL/day so going to ~85mL/day should be fine.
Wish me luck... lol