[kien]

What do you have in terms of livestock (and/or corals) in your 9 gallon display?
What are you lighting in your sump? You mentioned there is just live rock in there. Live rock doesn't need light unless you are trying to grow something? In my opinion Nitrates do not need to be zero to have a healthy and thriving saltwater system.

The magical product these days for reducing nitrates are "NP Bio Pellets". Do a search for them either in google or canreef search and you'll find lots of info. Your system would work well with a TLF phosban reactor with some NP Bio Pellets tumbling in there.

Your other options would be running a refugium with macro algae like Chaeto, and/or growing Mangroves. There's nothing wrong with these two methods, in fact they can work quite well, although I am not a huge fan of these approaches so I'll leave it to the proponents of those to chime in.