It works well on the intake of a filter, keeping your main filter cleaner longer and preventing any small fish from getting sucked up into the filter. You may have to use a bit of tubing to get it to fit okay.
it fits pretty loosely on my aquaclear50. It falls off during water changes if I bump it
I ordered a small pre-filter sponge. It is wonderful. I have several canister and power filters that it works perfectly on. Fits great. I will probably order more over time.