A biowheel provides surface area for bacteria to grow on. The bacteria break down ammonia that is introduced to the tank from fish waste, fish food etc. One group of bacteria breaks down ammonia leaving as a byproduct nitrite, which is toxic to fish and invertebrates. Another group of bacteria will break down nitrite and leave nitrate as a by product. Yet another group of bacteria can break down nitrate (not terribly toxic to fish) and give off nitrogen gas as a by product completing the nitrogen cycle.
Herein lies the problem with a biowheel. Bacteria on a biowheel are very effecient at breaking down ammonia and nitrite. However, a biowheel does not provide the low oxygen environment needed for denitrifying bacteria to grow and break down nitrate. Thus, you end up with a very effecient nitrate producing machine with little to no means of nitrate export. The end result is high nitrate levels and possible algae problems.
The benefit of using LR
for biofiltration over a biowheel or other similar product is two fold. The LR
provides even more surface area for bacterial growth. In addition the porous nature of LR
provides areas that are low enough in oxygen for the anaerobic denitrifying bacteria to grow (as does the deeper layers of a DSB
) With proper flow, the bacteria living deeper in the LR
will break down nitrates and keep nitrate concentrations lower.
The high nitrates are not much of a concern in FO
systems, but inverts and corals are not tolerant of constant high nitrate levels. If you're only keeping fish then there is probably not a problem keeping the biowheel running. I've been running my 20 gallon tank with only LR
and a skimmer for over a year now and have kept nitrate levels undetectable by my test kit (Seachem) HTH