Well I'll toss in a thought or two here.
A typical magnifying glass is like a lens, thicker in the center and gradually thinner glass as you move to the outer edges. Light entering the lens at the center passes straight through, where as light entering the curved part is bent or refracted towards the middle. In a "round" lens, the light rays are refracted or bent towards a focal point some distance from the glass lens. This is where all the rays of light will "meet" together. At this point the intensity of the light is at it's greatest value, it's where the light rays all converge together. The distance of this focal point is determined by the thickness and curvature of the glass lens. The amount of light would be more "intense" as you near this point, some distance from the glass lens, but to what degree I don't know.
You'd need a pretty large lens or magnifying glass on top of your tank, one which would be hanging out over the edges of your tank a considerable amount, and it would have to be round, or at least the part that is bending the light rays would have to be round.
I suppose it's an interesting idea, using something like this to increase light intensity in a reef tank, but I don't think it would be too practical. Plus the chance of burning corals always would exist.
It reminds me of being a kid on the playground and playing with the magnifying glass on a sunny day. Those poor ants
Anyways - just thought I'd reply to your post - probably not much help.
See ya - Brian
