I run my halides 5 inches from the water. The lower the better in my opinion. One thing that everyone is overlooking here is water changes. Your doing 18 gallons per week? That is 10% if you had nothing else in the tank. If your tank is loaded with rock then you are doing more like a 20% or more since the rock is displacing a lot of water. If it is 180 gallon tank full of rock you probably only have 80-90 gallons of water. Times that by 4 weeks and your doing 80% per month! I used to beleive it was helping my tank by having such perfectly clean water, but I couldn't have been more wrong. I lost a lot of the mushrooms I had, most corals shiveled up to near nothing and sps turned brown. Even the anemones didn't like it. At the same time I had a tank, that I neglected, (IMO), and after scaping the green off the glass I realized it was exploding in life and growth! I learned that too much water changing can also be a very bad thing. I quit changing so much water and everything came back. I don't even do water changes on the prop tank any more other than making up water as I sell frags so it realistically gets about a 20% change just from selling frags. I did up grade the tank system recently to the Norris prop tank so it got a small water change. What I am trying to state here is: too much water changing can also kill your livestock. Stick to about 3-5 gallons per week and see if that helps. Maybe not even do a water change for several weeks before you start the new schedule.