The tighter the optic (smaller the degree), the higher up you can mount them. You could hang LED's with 120 degree optics a few inches from the water, or 10 degree optics several feet above the water.
As others have said, to make a proper recommendation we need to know tank size and lighting fixture.
If it's a DIY fixture, you could sketch up a scaled print and determine the output of each LED by drawing cones from each emitter down to the tank: google sketchup is a free software program that will allow you to do it digitally. Otherwise, you'll need graph paper, a ruler, calculator and protractor.