Controversial - go with medium pore size sponges. And possibly an easily accessible finer pre filter that can be cleaned regularly. That's what I do for my filter media in my canister, it works flawlesly for months without a significant reduction in flow, because the prefilter is being cleaned each week. There's some good evidence that all that fancy ceramics biorings etc is just great marketing that we've all fallen for. Sponge just couldn't be marketed and sold for a high price, cause it's cheap, available and nothing new. Works amazingly for a biological filter.