for ottos, wait until you actually HAVE algae. they are very fragile when introduced to a new tank and they need their natural food source immediately to start settling in. 20 ottos is a LOT. they are little piggies. i'd shoot more for like 5 or 6.
My experience with ottos is they die off for one reason or another. I had 12 ottos in my 20g tank and I ended with 3 after all said in done. I've had the 3 ottos for over a year. So I picked 20 because I figure I'll end up with half.