Can I grow algae on a red shale rock?

Shade89

Member
I bought two red shale rocks from Petco because I'm trying to grow algae for three of my Oto Cats. I put one of the rocks in a tray outside with dechlorinated water and fish food but there wasn't much sunlight today. Just curious if algae will even grow on this type of rock or if I should try a different approach to this.
 

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FinalFins

Member
My approach when I grow algae rocks-

If the rock is smooth, it grows algae.
 
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