I will assume since you have 48" lights, the tank is 48". 1 150w MH
fixture will only give you coverage for 1/2 the tank. There will be a huge difference in brightness between the aprox 2' area lit my the MH
and the rest of the tank. You shouldn't have any issues with heat causing the glass to break, but you will probably have issues with salt creep and splash marks on the glass cutting down the light penetration. Pretty much any water that hits the glass under the MH
will dry instantly.