Couple of things:
1. The filter floss you mentioned above ... well, it can actually turn into a nitrate factory if not kept clean. I have some in mine and I make it a point to clean it out every other day or so otherwise it gets brown and gunky, which leads to nitrates shooting up.
2. Bioballs ... same as above, if left to become gunky, they turn into nitrate factories as well. IMHO you'd be better off replacing the bioballs with LR rubble. LR works much better for mechanical filtration than the balls.
3. Do you have any room to add some macro algar like Chaetomorpha? If so, that will REALLY help in reducing your nitrates as they eat up the nitrates.
Cheers!
Yash