Nope, no prefilter for me on the FX4, FX6 or the SunSun 303B. I see no need for something like this on a canister filter like these that can hold such a large quantity of filter media. It would be a different story if I was running a small HOB filter. Then I could see that the pre-filter would be a benefit. I have all of my canisters setup the same way and in this order. First tray or first media the water passes through is layers of sponge filters in coarse, medium and then fine. The rest of the trays are packed with bio media in the form of BioHome Ultimate and these filters can hold a lot of it. I only use any type of Chemical media in one filter and I use several media bags filled with Purigen. This keeps my water parameters in check and water changes are only required every 2 weeks or so when Nitrate levels reach 20 ppm or slightly higher.
Later
NCH