I'm in the process of setting up a 125G 6 foot mixed reef. I plan on running AI Primes over it mostly due to cost and the fact that I was had good success with them in the past. But I am debating whether three or four would be better. It is going to be a mixed reef but more likely leaning towards more LPS and softies. Maybe a few SPS higher up in the tank. I think three would be fine.