grow rocks float!

[Michael]

Greetings! I made your home-built garden (adapting it to my space for eight (tomato) plants. I bought Geolite grow rocks for the medium. My pump works fine, raising to the level of the out-flow pipe, but there's a problem: the grow rocks float! They go up as the water fills the pop bottle halves, then go back down as the water empties. Is this going to screw up the delicate hair-like roots of my (tomato) plants? I was under the impression that once planted, the roots couldn't be moved. Help!

Michael in Durango, Colorado