I see a basic misunderstanding here Jayne. We only change media in our filters when they are quite literally falling apart. I clean media whenever it begins to plug up and reduce flows but I am using the same media in most of my filters that I was 5 or more years ago. I have never used a "carbon sponge" in a filter but it sounds like it is probably nothing more than a sponge filter with some carbon embedded in it. If that is the case, it can be treated the same as any other sponge and just be rinsed and re-used. Changing any media at a mere 3 months is a prescription for never truly finishing a cycle. I am sure the filter manufacturers would love that but it will not help your fish.