Lighting a 120 48x24x24 help?

[XSiVE]

I'm seriously considering switching from my 2x250W Halides + 2x54W T5's to LEDs on my 120g reef. I keep SPS and LPS with a few other random things.

I was thinking that 4 rows of 20 LEDs would work okay... mixing cree XRE Q5 and XRE RB with 60 degree optics..

It seems like Mike got a ton more PAR than what Im looking for with his setup at a similar density, but with 40 degree optics, but using a dimmable driver I could probably dial them in to where Im looking for.

Am I on the right track?

also, Im not really sure about drivers.. I guess I should take a quick refresher course in electronics

[XSiVE]

Okay so I realized that geometry could help me solve my problems of wondering about coverage..

if I exclude overflows, I need 1152 square inches to have 100% coverage at the surface of the water. A single LED with a 40 degree optic should give 41.39 square inches of coverage if it is 10 inches above the water surface... to get full coverage I would only need something like 27 LEDs

Of course, I'd need some overlap so I guess Im safe with somehting like 72 or 80 Does this make sense?

[evilc66]

You almost had it. 20 LEDs per row is about right, especially when you take the center brace(s) into account. I think you will need more than 4 rows though. With the tank 24" wide, that's a lot of area to cover well, and more like 6 rows is often necessary. If you put row spacing at 3" (my normal recommendation), then you have 15" between the first and last row. This leaves 4.5" front and back, which the LEDs will cover nicely, without much in the way of light spill.

[XSiVE]

Thanks for the response Evil! and wow you're close by! That sounds somewhat similar to what I was thinking about.. going to 6 rows

After thinking about it for awhile, the 72 number seems nice since it would only require 6 drivers... but stepping up to 8 wouldnt be too bad if I wanted to go to 96.

for the 72 approach I could place two 6x6 grid pendants with 3" spacing on LED center, 10" above the water with 60 deg optics.. each LED should cast a circle of a little over 11" diameter on the water surface which also means that it should fill in (5.7" radius) the sides pretty well.

My concern with this approach, however is that I may end up with a dark area in the center of the tank where the two pendants don't overlap and / or the brace interferes. I could move them in a bit but I would probably be hurting the outer edges.