I currently have 2 54l eheim tanks for my sailfins and their fry inside each is an eheim 2007 filter with the sponge inserts the tanks took a long time to clear when first set up and im not sure they are working as well as my other filters (one external and one internal) i wondered if i could cut the insert and place some charcoal or other filter media in their to impore things ive seen some of the home made filter suggestions and thouht this might work?