I agree with the "not enough live rock" comment.
If you can't buy more live rock to stock your DT / Sump properly, you could use a canister filter. It can be a nitrate factory but at least it is very efficient at oxidizing ammonia so long as your water is well aerated. It can serve as a GFO /carbon reactor at the same time. Just a thought. Probably best to bite the bullet and buy some more live rock though.
Canister filters don't have anaerobic areas for denitrification to occur, so your water chemistry would certainly benefit more from live rock.