rockwood and lavarock/clay pellets work great for hydro. but all will grow algae if left in the light. just had this prob in my system, easy fix. i use mesh pots with the medium i just talked about and covered with a pot lid same size as my pots. actually ordered them together as a set. anyway try some mylar, white plastic, lids, anything to keep the light off the tops of your pots. also check your other parts of the system, you shouldn't use clear tubing or im my case white buckets. i covered my buckets and cleaned everything nice and finished my grow. next time around had no problems. hope that might help.