Box filters are a type of mechanical filtration system that is commonly used in koi ponds. They work by passing water through a series of filter media, such as filter pads or filter foam, which are contained within a box-like structure. The filter media helps to remove physical impurities, such as dirt and debris, from the water. Box filters are often used in combination with biological filtration methods, such as bead filters or sponge filters, to provide a comprehensive filtration system for your koi pond. They are relatively easy to maintain, and they can be customized to meet the specific needs of your pond. Box filters are a popular choice among koi pond enthusiasts because they are effective at maintaining water quality and they are relatively affordable.