advice I am just about to purchase a led unit

Hello,
I have been looking at leds for my 120 gallon tank for 6 months now, i look at diy and for a while I thought I was going that route but my husband is so busy I decided to buy a unit. I am gonna buy the Orphek pr-156.
My question is mainly the degree lense I just want to make sure that I am understanding it.
120 degree lense is wide and would be for shallower tank
and 90 degree lense would be for deeper tank.
my tank is 48x24x24 so would 90 be better and why?

Also how far from the water should I hung my fixture, I have a canopy would that cause a problem?

 

I suggest you ask these questions of Orphek since they are sponsors here.

Your ideas on the optics are correct. You may have problems depending on the height of your canopy. I would say you would want the unit about 12" above the water.

 

The degree of the lens is then just dependant of the size of the tank. The optic will dictate over all par and intensity of the light. It can also dictate the heighth of the fixture above the tank. Here is a great sight that can explain optics. http://reefledlights.com/how-to-diy-led/ and on more...http://www.ledgroupbuy.com/pages/Choosing-the-right-optic.html?fullSite=1

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Go with the wide.

Pretend this is a 400W halide (what it is supposed to replace). If you put a very spotlight effect on a 400, it would fry a 24" deep tank.

Orphek says that you only need the 90 degree lenses if you go any deeper than 30 inches.