Like Biffer indicated bio balls only do so much. In a trickle filter bio balls actually work too well. They work great at turning ammonia to nitrate. They do that much better than live rock or deep sand beds. However all that nitrate stays in the tank. Skimmers do not skim much out in the presence of a trickle filter, so they remove little dissolved organics as the trickle filter turns them to nitrates so efficiently and quickly. Bio balls in a trickle filter are good for really dirty fish only tanks such as predator tanks where the elevated levels of nitrate are tolerated by the fish. Not a lot of denitrification is done by live rock, surely no where near enough to not do water changes in most reef tanks, but at least the skimmers can remove dissolved organics before they are converted to nitrates as happens with a trickle filter. There are also problems with a scum layer of dissolved organics adhering to bio balls that does not happen with live rock or live sand. Bio balls underwater in a sump are less efficient at converting ammonia to nitrates and they accumulate the scum layer which sloughs off spiking nitrate levels nearly instantaneously.
