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In general, having the light to close to the top of the tank is not a good thing. A light that is up high off the tank will give a much more even light distribution across the tank.

Lets say the light is 3 inches from the top and the tank is 24 inches deep. If light intensity falls off linearly with distance, then 3 inches under water will be half the intensity as at the surface. 6 inches is 1/3 the intensity 9 inches is 1/4th intensity of the surface, and at the gravel it will be 1/8th the intensity as at the surface.

If the light is sitting 24 inches above the surface, then the intensity at the gravel will be only 1/2 the intensity as at the surface. So it is much more uniform throughout the depth of the tank.

You will be much more likely to get algae in the upper portions of a tank due to high light conditions if the lights are sitting on the tank.

I raise the lights up for this reason as well as it just makes it sooooo much easier to service/clean the tank if you arent continually bumping into a light fixture.
 

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Not saying you need to do anything. Loads of people have lights sitting low down and have no problems. I'm just saying in general, its better to move the lights up higher to provide a more uniform light level.

If you start seeing algae in the upper levels of your tank, its something to keep in mind.
 
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