Many people confuse a prefilter with a biofilter. It’s easy to do. Those who manufacture prefilters often call them biofilters on the packaging. They shouldn’t be able to get away with it but they do. Prefilters are used in conjunction with submersible pumps to strain large debris so that it won’t get caught up in the pump. They’re often built onto certain pumps or come as optional attachments. We like to construct our own with pump baskets.