It will help to bear in mind the Denitrification process.
Nitrites to nitrates is an aerobic process
Nitrates to nitrogen is an anaerobic process
When the process is in balance there is never a build up. bio-wheels are not the problem, Overstocking is the problem. More wastes being produced for what the tank can handle. If they did not have bio-wheels in on their filters they would have a build up of nitrites or ammonia. The system backs up at the weakest point. As Lando points out one solution is to increase the amount of anaerobic bacteria with LR
, the other is to reduce the stock to what the tank can support.