I wonder on its cost effectiveness, I'm guessing it won't be better than anything else in a few months. If pumice is cheap enough though, you could simply toss and replace like filter floss.
If washed thoroughly I doubt all the internal suface areas will be blocked. Making it still better than say plastic bio-media which is limited to surface area only. Costwise, if you can reuse the stuff again and again, it's a great buy. For those living near volcano active regions you can pickup pumice stone for free. Can't beat that for cost effectiveness