In general, no 2 Bio filters are alike. So that is like saying how fast is safe to drive through your neighborhood?
If you use the so called "skippy" filter, it is a short oval shaped feed and watering vessel that has scrubbie like material in it and water passes through at a given rate.
The 55 gallon bio filters, I am going to call, from now on the "Doc Filters", They are 55 gallon drums with cut irrigation tubes, packing straps, or turned PVC and the water goes through slower because of the long climb to the top.
That said, it depends on whether you are using the Skippy or Doc method.
The Doc method depends on gravity to settle out the solids and they are trapped in the sump. So the water must travel at a rate fast enough for the volume of pond to get cycled yet not so fast that the gravity effect fails to work for you.
When I examine the water coming out of my overflow, it looks like bottled water. That tells me I am not pushing it through too fast since my pond is also clear.
As for the anerobic bacteria, if you have enough bio material, be it the Skippy or Doc type media, it will do it's job.
Remember, the clarity of the water is not due to the bacteria, but to the gravity. That is why I feel the 55 gallon drum is the best bio filter out there.
So the answer lies in trial and error. If I were you, I might start with 350GPH if your pond is small, or larger if it is over 1000 gallons.
I use a Beckett W1150 in my 1400 gallon pond and it works flawlessly. You can get these at any Home Depot.
Hope this helps. Remember skippy is a particular method using sponges, so the name does not apply to all bio filters.
