You mean for the inside of it when I'm using it to vacuum? Fairfield PF-5 Poly-Fil Premium Fiber https://www.amazon.com/dp/B002OUP6JE/ref=cm_sw_r_cp_apa_i_SlC8Cb65KAWJ3 I just pack enough in that it'll catch the detritus. It's cheaper than what they market for pets. And I suppose you could use it a few times as long as they weren't far apart but I wouldn't leave decaying detritus in the for more than a week lest you want to dump nitrates into your tank. I think is easier to do a thorough clean once a month then trash it.