Quick or slow water flow in the filter?
What would you recommend? Slow water flow (minimum pump performance ensuring the bacteria that fixate nitrite and nitrate have longest possible contact to the water inside the filter medium), or a normal to high flow rate ensuring the water body is well provided and "blended" with nutrients?
in fact, a cross of the two would be optimal - in case of high nitrate pollution, this can be achieved by a combination of a 'conventional' biofilter (typically with flow rates in the range of several hundred liters per hour) and a special nitrate removing slow flux filter with flow rates of a few liters per hour. This ensures proper pollutant oxidation to mostly untoxic nitrate within the biofilter, along with good oxygen enrichment and water agitation, while the slox flux filter breaks down the formed nitrate into harmless nitrogen gas, which then escapes into the atmosphere. A compromise is possible in so far that filters equipped with sera siporax (recommended: 1 liter sera siporax per each 100 liters of water) and moderately high flow rate (up to approx. 400 l/h) rather good denitrification will take place also within the biofilter.
Dr. Bodo Schnell