With marine tanks, you can rarely go wrong by adding more water flow. If you've ever gone snorkeling in a fringing reef, you'll know what I mean. There is a constant movement of water. While I currently do not maintain any marine tanks, in the days when I did, in a four-foot marine tank, I would typically place a Penguin 1140 powerhead in the back left and right corners, as well as use two Penguin 330 Bio-Wheel filters, one on the back left, and one on the back right. This would provide sufficient water movement for anemones, corals, etc.