I was told BioSpheres will be less likely to generate that brown mulm on the sandbed that is often encountered said:
Since my 400g has been taken down (April 2012) and all the livestock was transferred to the 215g with some live rock and about 80lbs of live sand, I've not tested nitrate once... until yesterday. I was curious what that number might be. It measured about 3ppm, not bad at all since I feed my tank 3x a day. The feeding replenishes what the biopellets would otherwise strip from the system, so you are able to fatten up your fish without concern. Be sure to dose bacteria regularly, and water changes shouldn't be omitted. I tend to do 25% monthly.
As with anything, water tests are key to knowing what is going on in your tank, and watching the livestock closely for any clues both good or bad. I hope this helps.