Recommended LED quantity for a given tank

This seems to be a popular question, and the rule I used to see a lot is figure one LED per every 10-20 square inches of surface area. With the advancements as of late and as more and more people begin to convert to LEDs, this rule is getting blurry.

So lets use me as an example.

My tank is 60x20x24, lots of LPS with some SPS up high. It's currently lit with 3 250W MH's.

I'm considering your dimmable premium kits (CW XP-G & RE XP-E). Would the 48 LED kit be sufficient?

 

I think you hit the nail on the head when you mentioned that things are getting blurry in regards to this rule. With the advancement of LED technology lumen output is steadily increasing to the point where many reefers are over doing it and actually bleaching coral instead of not providing enough light.

Determining the optimal number of LEDs is still (and probably will always be) an art and not science. The reason for this is that there are so many variables that change from tank to tank, such as the type of coral being grown, the angle of lenses being used, the distance above the water the lights are being set-up, the amount of light the corals are used to, etc. What we have measured is that one of our 24 LED kits (a standard premium kit using Cool White XR-E's and not XP-G's, which produce about 30% more lumens than the XR-Es) produces greater PAR levels than a 250W MH. Hence, using this info and checking with the one LED per 10-20 square inches rule, it seems reasonable to say that you would probably need somewhere around 72 LEDs for a tank your size and with what you described as being inside. If you had a lot of SPS you might consider going up to 96 LEDs, but given the current status of the tank that would probably be overkill.

 

good question and answer, i have been wondering about the same kinda things!

 

Ah, I guess I did leave a few details out, and I do realize the number is a bit of an art, but nevertheless...

First, I mis-spoke and said the premium kit instead of the ultra premium kit. I was attempting to account for the fact the XP-G's are brighter than the XR-E's (as you said) in my "guesstimate". I plan to use 80 deg optics on the RB's and 60 deg optics on the CW's. Lights will be hung above the tank, so distance off the surface is completely open, but I'd probably shoot for 8-10 inches away. I will say my 250W MH's are a bit too strong sometimes as some of my corals on the sand or near the bottom take a bit much light for what they're most happy with.

Also, there is a local reefer in the area that converted to LEDs from 250W MHs about 4 months ago. He went with 36 LED kit (50/50 mix of CW/RB, all XR-E) and is only running them at ~70%. Any more is too bright and will bleach things. He says if he had to do it over again he'd only go with 24 LEDs. Granted, his tank is 36x18x20, but I figure this should still be a rough estimation basis.

Does any of that info change your answer? (and thanks, btw)

 

If you're going with the Ultra Kit with the XP-G's, and you feel like you might have a bit too much light in the tank already then I think a 48 kit will work out well for you. With the XP-G's 24 LEDs would give a good amount more PAR than a 250W MH, so one of the 48 kits would probably fall somewhere in between 2-3 250W MH lights (which sounds like the right amount of light for you).

The great thing about building a light fixture yourself (other than the money savings) is the flexibility it offers. If you find out that 48 LEDs isn't quite enough light you can always add another 12 kit (or something similar) in the future with (hopefully) just a few minor mods to your original unit.