The higher the light is above the tank, the better it is able to cover a tank that is 2 feet longer than the tank. A high quality 4 x T5HO, 4 foot long light would need to be about 30-32 inches above the substrate to avoid having too much light to easily live with. At that height you might be satisfied with it for coverage of the full length of the tank too, but the ends of the tank wouldn't be as brightly lit as the center.
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Hoppy
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