Hi,, U could get some plastic cut a slit from one side to the middle and place on top of the rock wool. This will stop the algae, when you place in the pebbles cover the rock wool with pebbles to stop the light as well,, if you dont it will attack the stem and die die die..

Hope this help Green boy
greenboy Reviewed by greenboy on . Rockwool Question The Cuttings are about 4 weeks old big and beautiful. The top of the rockwool has a wierd dark green look, not all of the top but it kind of branches out. Any ideas what it is. Its a drip system, if that helps. Rating: 5