I've got a 21" version over a roughly 20"x20" tank it's 18" deep. It's currently set at 35% intensity max. I have 2 hammers 1 torch 2 Duncan a few zoas and 5 rfa no coral is past the bottom 3rd of the tank. All are happy and healthy. I have no idea what the par is, I don't have a meter. The light is dead center on the included tank mounts. Most corals are in directly below the light but the he torch and 1 hammer are more towards the back and 2 of the rfas are touching the front pane. These are more powerful then people give credit. I'd think 3 would be more then enough on a standard 125 probably over kill. Not sure at the ends of the tank may need to keep low light or no corals in the last 3" on both sides. My light goes wall to wall. I get a decent algae dusting on the sides after a few days probably could have used a 12" for my purposes.