There are some good vids on youtube about the blue foam and how to use it....it is marineland size U.....some of the other marineland filters are made different and even out of diff stuff...so make sure you get the size U. I got mine at Pet Supplies Plus....PetsMart has it too, but they were out when I bought mine.....so any pet store that carries a nice selection of fish supplies should have it. It DEFINITELY does much better IMHO than the polyfil that comes in most carts.