Yes the Hydroton float in water therfore I would put your rockwool seedlings in a plastic pot and fill with hydroton. Set these in your tray and do not let the water go above the top of the pot only high enough to hit the roots. I use a DWC system and have great success with it. My ph seems very stable with this method . I hear ebb and flow ph flucuates more. Ph is very,very important for nutes intake. This must be maintainted at all cost.