There is confusion about filters. What size of filter should I buy? If it is a BIOLOGICAL filter, the bigger the better. Often the filter will have a gallon rating . This is probably the WORST rating system because it is not clear what the function is. The function of a biological filter is too contain BIOMASS - decaying matter. Each fish and plant produces biomass as part of it's lifecycle, falling leaves produce lots of biomatter and are a frequent factor in overloading all types of filters. Simply put, the larger the biological filter's capacity is, the less difficult caring for the pond will be. If the biological filter is not large enough, you will not be able to clean the pond without removing needed biomass. REMINDER - a certain amount of biomass is required to maintain a balance environment! Wouldn't you rather have it neatly stashed away in a box? Then you will be free to clean down to your pond's liner.