Originally Posted by atom

4x is just nothing more than a guide. You can have 10x but you must have large sump with sufficient biomedia to compensate for the reduced retention period. I prefer high turnover. 6 to 8x will be nice.

An exaggerated example:

The water speeding pass a chamber containing CR at 1000 l/hr, can remove 4 mg/l of ammonia.
When the velocity increases to 2000 l/hr, the chamber can remove 2 mg/l of ammonia.
To compensate for the losses, the water has to pass 2 chambers in order to remove 4 mg/l of ammonia.