Hi Sue. Most that are knowledgable about this believe a submerged biofilter should be at least 10% the size of your pond for a full fish load. The reality of it is that it should be sized to your fish load. I raise koi so I try for at least that. If you have a light fish load you can get away with less, but it never hurts to have more. Because the bacteria needed is O2 dependant you can get away with a much smaller area if you use trickle type bio filter. Have seen many people using less than 10% in submerged bio filters but with high turnover rates. Usually at least twice the volume of the entire pond per hour. One thing I have noticed about trickle type filters is that they cycle much faster and as long as the water is pre filtered there is little if any maintenance other than cleaning the pre filter. Adding bubblers to the bottom of submerged bio filters increases thier capacity too.
