Growing algae

[giffmastaflex]

So I am trying to grow some algae on two rocks, the other day I put them in a bowl with old tank water and some plant food and have been letting it sit in the sun for two days now. How long will it be until the algae is visible on the rocks? And is there anything else for me to do?

 

[Sharkfin]

My old tank used to have a whole bunch of algae all over one glass where they sun light barely reached, the nutrient level was very high. So try to increase the nutrient level in the water and we dont actually need a bright light from the sun to make it grow cuz they can take whats in the water with even less light than plants