I find that these work better if replaced after every tank cleaning. (I have a turtle, and because the filter is so powerful, I wind up cleaning his tank maybe once a month.) So far I've reused the sponges twice, and I notice that the tank gets dirtier sooner. So yeah, definitely replace these and they do a spectacular job!
Works well for the type
Review Date: November 21, 2006
Reviewer: Joshua M. Guillemette, Orlando, FL USA
This is simply a foam filter that removes most refuse from the water. It stays operational for a fair amount of time considering the volume and number of critters. While other filters are more effective, here you get what you pay for: a simple, easy to clean filter that gets rid of most of the "yucks".
Pretty awesome