I agree to a certain extent that the bio-balls will trap particulates thus raising your nitrate level.The thing is,LR rubble will also trap particulates just as easy.The reason wet-dry/bio balls cause elvated nitrates is this.Bio-balls are a place for aerobic(oxygen living) bacteria to grow.This bacteria is what converts ammonia to nitrite then to nitrate.So,you end up with a filter that is constantly producing nitrate.In order to have low nitrate levels in a reef tank,you need to have a balance between aerobic and anaerobic(low to 0 oxygen) bacteria.Anaerobic bacteria are found in the bottom layers of the sandbed and the inner pore structures in LR.Aerobic bacteria are found on just about every substrate in your tank.When you add an external "biological" filter such as a wet-dry,the huge amounts of aerobic bacteria they grow cause an inbalance with the amount of anaerobic bacteria in your tank.Since this anaerobic bacteria is what processes nitrate,they can no longer keep up with the levels of nitrate being produced by these external "bio" filters.