Hi J, Clay pebbles are OK only if you use a small amount please make sure that you pre balance them first as in reality they are a building aggregate from Norway & Finland and are very alkaline. This would be the same advice I would suggest if you used the Wilma system, pre treat the pebbles. If you decide to use the easy2grow Kit with pebbles then only fill to a depth of around 4-6 inches. Place the plant on this level, back fill with more pebbles and top water for a week and then turn the system on. Hope this helps, bye for now Josh