This was discussed earlier, but I did not gather a conclusive information what can be used instead of filter cartridges to save money. I bought Auaen 20 power filter for my 29 g tank in April and did not replace cartridges since then. Now, I see it circulates the water, but does not filter well any more. Can I use some cheap sponge pad insead of relatively expensive cartridges? What kind of sponge would be th ebest: May be I can use the same sponge again and again after sim0le rinsing it?