Hi Pandagold,
Liverocks are the same ones in you use in your main tank. However, since right now you are lacking in anaerobic bacteria housing, one of the solution is to have large piece of liverocks. The reason why you need large pieces (approx 5-6inches) is that smaller pieces will not have anaerobic zones.
If I were you, I will remove coral chips vs bioballs if I suspect that its is the detritus accumulations that causes of my high NO3. This is because bioballs have smooth inert surfaces that do not trap as much detritus as compared to coral chips. Also, corals chip does not really act as buffer in marines set-up, this is due to 2 basic reasons:
1) Organic films (Bacteria colonization) and detritus build-up on the surface of the coral chips preventing direct contact between coral chip surface and the water.
2) Coral chip will start buffering at pH 7 or lower (Marine Tank optimum pH - 8.1 - 8.4). Thus, by the time your pH reaches the coral chip melting point, your tank could already have crashed. Reefers who uses Calcium Reactors will know, calcium media will not melt if you do not introduce CO2 into the CR chamber to lower the pH to 7 or lower.
Coming back to bioballs and corals chips, sintered glass or even liverocks in sump to increase the anaerobic zones. All of the above will work well if you have a good pre-filter (mechanical filtration), this is why you find reefers placing polyester wool to trap particulates prior to the bio-filtration chamber of your sump. If you do not have or maintain a effective mechanical pre-filter, you will still not get effective bio-filtration regardless of which media (bioballs, corals chips, liverocks etc) you changed to.
Now that you mention the fishes you are keeping, can you give us more info? The number and sizes of the fishes and your feeding regime will help provide more clues to your high NO3. Also, do you have a skimmer and whether you are getting sufficient skimmate may be another reason.
Like some of the bros here have advised, try to be prudent and do not jump into any conclusion without exhaustive investigating your actual reason for the high NO3. Do drastic changes will do more harm than good.
HTH