Hopefully someone will pipe in here with their own observations. But, from a technical point of view, here is the basics of antenna transmission (i.e. LED
light transmitting antenna, AKA LED
The lens is rated on the -3dB point, or where half as much light is transmitted as compared to max, or directly under the bulb. So it's not like nothing is transmitted past the 90 or 120 degree points, its just less than half of the peak. But lets aim for the -3dB point.
You tank is 28 inches wide and the fixture is 8.5" that leaves 20" total or 10" per side.
A 90 lens is 45 deg either side of vertical. In a 45 deg RA triangle, width = hieght, so about 10" high would give 10" to the side. But the LEDs aren't right up to the edge, so a couple more inches would give better coverage. 12-13" might be just right.
A 120 deg lens is 60 deg either side, which is a 3-4-5 triangle. So hieght needs to be 3/4 of optimal width. 3/4 of 10" is about 7.5", so at 12" you might get a bit excessive light spilling out the top.
Now, who's got what lenses on their Tao's and hung at what height? I am also curious because I will upgrade sometime and I plan on a 120 and 90 deg lens for my sloping structure.