rockwool
 

HydroRon I dig your site. I built a 9 bottle (2 liter) pvc system. It works killer. Thanks for the plans. Heres the question. Since the cups are circular and the cubes are of course square the rockwool doesn't fill the lower portion of the cup. Should there be air in the bottom of the cups or should the rockwool fill the cup? Also how high should the nutrient rise in the cups?

Thanks stork

You can take a few rockwool cubes and brake them apart to fill the bottom of your cups but wet them first because you shouldn't breath any rockwool particles.
That's the reason I use only grow rocks.
Because rockwool acts like a sponge I don't think the water would have to rise very high to work My guess is about 1/2 inch up your cube.
With grow rocks the height should be about this high.
Ron