Lynn, i just saw this. i think the hydroton would actually be more beneficial to the biological filter. they may be smooth and round, but i think it's what we can't see that makes the difference. i would venture a guess to say that it really isn't as smooth as we think. i believe they are covered with microscopic holes from air pockets in the clay as it was being dried. leaving a really nice amount of surface area for bacteria to grow on. not as smooth as the rock would be. i know microscopically, the rock would be textured as well, i just think the clay would beat it out.