I think it usually grows naturally, depending on how new your tank is. unless your tank has plants in it that outcompete all the algae for food. How old is your tank? What size is it and how much lighting do you have? What are your water parameters (pH, Nitrates, Phosphates, Ammonia). How many oto's are you planning on? I have 4 in my 30G. They cleaned up most my algae in about 2 weeks, and I had a fair bit to begin with.
Colin
