Here's what I would suggest:
1. 50:50 mix of cool white and royal blue LEDs is good. Mixing regular blues or other colours in there probably doesn't really make a huge difference. Red emitters look ugly and I have experimented with greens and violet and they look interesting on their own but don't add much when used with the white and royal blue.
2. For a 24" deep tank and seeing as you are replacing 400W MH you will likely want to use 40 degree optics to achieve similar PAR at the bottom of the tank. That means using more LEDs. You asked about a 48" array which will only work if you have little on the ends of your tank? How is your rock etc. set up? Do you have clams and SPS all the way out to the ends?
If you want good PAR coverage over the whole tank you will need to go closer to the ends. Generally you want to space your rows front to back 3 inches apart and you can leave 4" or so from the end of the array to the glass. In each row with 40 degree optics it would be best to space LEDs 1.5" apart but you could go up to 2". 2" might cause a bit of spotlighting in the first couple inches of the tank depending on the height of the fixture.
So if it were me I would try to do 6 rows of LEDs (you might get away with 5 rows if you have more open space in the front or back of the tank). Each row would have 36 LEDs spaced 1.5" apart. That works out to 216 LEDs and along with drivers that will be pricey. But to give similar performance to 400W halides there is no cheap solution.
3. Look at the Meanwell drivers. They are more reliable then buckpucks which tend to burn out if they overheat. They also run right off 110V AC rather then having to have a bunch of 24V DC adapters for the buckpucks. So for 216 LEDs you are probably looking at 18 Meanwell drivers.
HTH to start.
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