Pumice stone (Erith DA8)

This is a random one but I'm looking for pumice stone it's used as a planter /flower pot topper or for drainage or simular Ialso used in the bath but not the ones I want, i want it to add to my aquarium filter as a bio growing area bacteria etcc any who if you don't ask you don't get I can go buy some but probably more than I need at alot of expense at garden centre