You don't need to "change" them very often, that is just a marketing ploy in an attempt to turn non-consumables into consumables. It's like changing your bio media. I would really thoroughly clean 1 or 2 of them every week or two. Wring it out in hot water for a while, let it soak then repeat. you're not going to get everything out, but that's ok. This way you won't kill to much of your bacteria population and don't have to buy new filter pads.