How to grow algae on rocks?

Does anybody have a definite idea about growing algae on rocks?

I bought two large plastic containers, probably 40 gal.capacity and placed river rocks on the bottom. I added scraped algae from another container, some green water and some fertilizer. Ten days later I have green water and some green algae on the walls. Both containers are situated outside in a fairly good light, but not in direct Sun.

When should I expect some green or black algae starting to colonize the rocks? Is the water depth a factor? There is some 12 in of water above the rocks.

I'm asking this, for I have some 14 Stiphodons in my tank, and I will expand my inventory with other Stiphodons, some of which are known to graze exlusively on algae. The ones I have, of the 'Rainbow' group, are happy with algae waffers, chopped black worms and the algae in the tank