I got a used aquarium from someone. The aquarium came with a filter. The filter does not have any cartridges in it. How can I replace those. I know that you aren't supposed to change them after you start an aquarium, but where can I find a set to put in the filter to start with? It is the kind that hangs over the side of the aquarium. What should I do? Does a sponge act the same way as a bio-cartridge? I already have the carbon filter cartridges to put in it.
