Tell me why I can't use MH/T5 lighting systems designed for hydroponics?

[pastout]

i have a sunlight supply t5 48 inch 8 bulb fixture that was mounted 1 foot almost above the water and it had tons of light comeing from it and i didnt find the reflectors inferior at all even compared to the tek fixtures ecspecialy because every bulb has an individual reflector also. this was on a 90 gallon