What I was saying was to cut the old cartridge to fit or make it smaller. I cut my old filter in half in my Whisper 10 when I replaced the filter. I then took the piece I cut off and stuck it between the Whisper 10 and tank wall. Just pushed it behind the filter so I wouldn't have to throw any of it away. All the filter media does is hold bacteria and any gunk that is floating around in the tank. You could just stick a big sponge in it and nothing else. It would serve the same purpose. I have found that the Whisper 10 internal filter doesn't filter very well any way. A lot of stuff settles to the bottom of the tank. It does circulate the water pretty well though. It is what I started out with. Later when you have a little money you could upgrade. Put a little back at a time until you have what you want to spend on one and then pick one. Truthfully I have found that all HOB
work. Some are just more noisy than others. My Whisper 40 did a good job of cleaning the water. It was just so noisy I couldn't sleep, watch tv, or pretty much anything. You may purchase one and not have a noise issue. Or if you are satisfied with the internal filter don't worry about it. Just buy what filter media you can afford and use it. You don't have to purchase their cartridges if you save the plastic piece. Just wrap something around it and put it back in.