Have a 40 with similar dimensions as you and my 350w is about 8" above the water which seem perfect. Had these fixtures for almost a year now. Par is about 450 at the water surface and 50-60 par at the sandbed. Thats with 40-45 percent on both knobs intensity
I just lowered it so the light spill is non-existent. It's about 5-6inches from the water surface. You think that is too low?