What height over the glass center brace should the lights be?

[checkmate0021]

I bought this system to replace the T-8's system I had and in 48 hours the center brace in the tank cracked with the new system on it ..48" 216W T5 Aquarium Light Fixture Reef Fish Tank Light.. I notice this system is a lot hotter when running the the T-8 set I had before. So my question is how high does it need to be off the glass in order not to worry about it cracking again anyone have an idea? It was about 1 inch before I now have it at 3. the water level is the same at about 1 inch below the glass. Also about the repair in case that might help on the answer. I had a pc of glass cut (14x12X3/8) and sandwiched it to the top of the broken PC with silicone and then Epoxied the ends to the tank and old pc of glass. I used marine grade for both adhesives. At this point I am worried to run the lights more then an hour at a time for fear it will crack the glass again. Also during this repair I opened holes in the top of the canopy to vent heat in hopes this will help. (pic's of repair, and heat dissipating holes attached)

Thanks for an info or help...