I have used these in tanks, and in pond filers, and had to regularly clean them (sometimes twice per week in my large cichlid tanks).
I'd just toss them on the lawn and hose them down, flipping occasionally.
In tanks where they didn't get too gummed up, I'd swish them around in a bucket of old water change water.
If you don't clean them, after being gummed up they add a ton of nitrate back in the water column as the gunk metabolizes.