Try this..Works very well.http://www.kensfish.com/spongefilterlinks.html
It works fine for Snowballs and RCS. Maybe with CRS it would be wiser to go with a filter that is finer, due to the lower numbers of offspring they have.i dont think the aquaclear foam insert is 100% shrimplet safe. I use the sponge prefilter and it clog up quite often too.