I have a 65g (36X24X18) and have one on each end. I have a Nero 5 on one end and an Ice Cap 2K gyre on the other. Both are turned down quite a bit LOL. The Nero 3 is a good size for the tank (I think I have the 5 set at like 35%). I think you might be able to get away with one gyre, but you'll get better flow with one on each end. It does depend on what corals too.