Whats a good type of rock to grow algae on in a freshwater tank. I want to grow green algae or diatoms for my ottos. I will get 2 rocks, one stays in the tank, one is put outside where algae grows. I'll probably rotate once or twice a week. Could i use pebbles found outside my house, or are they too dangerous. I dont mind them lowering my buffer capasity cause the fish i have like a low Ph.