You can use a glass canopy with halides. Glass acnpoies hinder the exchange of carbon dioxide with oxygen at the water air interface due to the restriction of fresh oxygenated air to the waters surface, but that can be handled by having the area in front of the backwall covered by light grating instead of covering the whole top with glass. Otherwise you should have a sump with a fan running over it and flow enough to have the water fairly turbulent. Also you will need to clean the glass regularly to get good light through the glass, especially if your using anything in your tank that will splash water up onto your glass or that creates bubbles which will burst at the surface causing salt water to get on to your glass. Perhaps you might just consider covering your entire top with plastic light grating rather than glass. Halide lighting is typically kept off the glass due to other heat issues, six to nine inches is typical when trying to stop halide light transference to the tanks water. If you look at halide lighting fixtures they typically have a glass sheet over them as a UV shield which is only an inch or so from the halide bulb.