Some old dead guy once experimented with light and water. Much was learned and it's all really boring. Two points come to mind that in part answer your question. First of all at the boundry between light and water a portion of the light is reflected. The amount that is reflected can be calculated using the sin of the angle of incidence ( or something like it). Basically the amount of light that enters the water is decreased as the angel increases. When light enters the water directly below it very little is reflected and can be said to be more intense.
The next most boring thing is what happens to the intensity of the light as it passes throught water. If I recall the intensity drops by about 50% in the first 10 cm's, and then much more slowly afer that (60% at 20 cm to 30% at 40cm).
So adding a light in the middle of the two existing would increase the intensity across the tank, and especially directly below it. So if this is what you wanted to do...
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