jay24k - I haven't found any so far. They don't leach so unlike the concrete rocks - no curing, soaking etc is needed. you have much more control over how they'll look as you can make them to what you like. Most will say they don't filter out as much but all you need to do is soak a sponge in clay and fire it. the sponge will burn out leaving behind a very porous surface - same as rocks if not better. Not many have done this with such a large system, so it will be very interesting in the years to come.