Tetra Pond Replacement Pad Set for Submersible Flat Box Filters helps to keep your pond filter in top shape. Contains one coarse filter pad and one fine-mesh filter pad to catch and remove debris and suspended matter from your pond water to help prevent your pump from clogging. Designed for use in the Tetra Submersible Flat Box Filter this replacement pad helps keep your pond clean and clear for a beautiful customized backyard or patio feature. To measure your pond volume (in cubic feet) calculate length x width x average depth; multiply the total pond volume by 7.8 to calculate the number of gallons.