I voted for sponge but actually use polly wool (pillow stuffing stuff). I put a handfull in my over flow and change out weekly. Cleans up water great and just throw away when dirty. Get it in big bags at local sewing store cheap. Just make sure it is not hypoalergetic and or mildew resistant as i imagine it has some sort of chemicals in it. LFS does sell it as well but it is more pricey
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