Werd.
Things to look for
brand of the diode - crees are the best but others are swooping in, especially in the lower NM and novel colour (ie not white and blue) range
Wattage of the diode - on a large system don't buy anything with less than 3 watt diodes.
Number of diodes/total wattage/arrangement on the fixture/lenses - this lets you know what sort of spread and punch you'll get.
colour mix of the diodes - standard for the past 5 years has been blue/royal blue/some kind of white (warm, neutral or cool). This works, but you might find yourself thinking of your tank as washed out if these are the only colours you get. There's umpteen nauseating threads on the various forums debating the merits or lack thereof of the various flavours of white diodes. Latest and greatest fixtures all have more colours than this, and DIY forums have been doing multi-colour units for years.
The number of controllable channels - the more the better, but the more the pricier.
After that things like heat control, diode lumen per watt ratings (cree keeps coming out with more and more efficient diodes), build quality, and control interfaces are icing on the cake that will make your light either pleasant to work with/last a long time/cost more than your first car. But for the bare bones purpose of growing corals and how they look while doing it, it's the first list that counts.
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