I suppose it would depend on whether or not you are using a splash guard on the halides. If you had some sort of protection for the lamp you could put it any distance away that you please, but if the bulb is unprotected, you may want to keep it where it is due to splash concerns. HTH
I had my mogals about 6 inches from the water. Cleaned them weekly to remove salt spray. I had no splash guard. The typical drop splash from a surface bubble won't bother the bulb but a full spash probably will shatter it. Heating the tank water is more of an issue.
I now run 250w HQI lights and I keep them 10 inches from the water surface.