Assuming your cold water fishes were goldfish, they produce a LOT more waste than smaller tropicals so your filter would clog up a lot faster - be it internal or external.
Never 'replace' the sponges in any filter, unless they are dropping to bits. Just clean in used tank water when the water flow decreases significantly. I have sponges in an AquaClear that are over 10 yrs old and still going strong.