The bigger tanks need more light because even in clean water, the strength of light is reduced with depth.

If you are not interested in plants, don't worry about the stuff below. :)

In a 20 gallon tank, it is estimated that when the light from a standard aquarium light gets to the bottom of 15 inches, it is only 1/2 of what it was at the surface.

There are still neat low light plants which wouold do fine there. Rock piles for mbuna or driftwood obviously don't need the light.

Deep tanks, especially for deep bodied fishes like angelfish are really showy. Talk to your LFS dealer.

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