Personally, I disagree that MH is still the gold standard for reef lighting. Period.
I've done both MH and LED, and I seriously doubt that I'd ever even consider using MH again. The coral growth I see under LED is as good as it was under MH, and with some corals it's actually better. The colors of my corals under LED is every bit as good, and generally much better than they were under MH. I keep various SPS including acropora, pocillipora, and montipora quite happily under LED lights. I also keep LPS including goniopora, euphyllia, trachyphillia, duncanopsammia, favia, acanthastrea, and blastomussa under the same LEDs. Zoanthids and discosoma love them, and the only problem I have with them is that they grow too quickly. The fish, incidentally, couldn't care less. But I think they must appreciate it when the light gradually comes on in the morning and slowly dims in the evening.
As we all know, MH lights are either on or off. They turn on, warm up for a few minutes, and then blast their way through the photoperiod at the same constant light level. With any high-quality LED fixture, however, one can easily simulate sunrise and sunset, clouds, storms, and the varying natural intensity of light throughout the day. Natural lunar cycles can be recreated with moonlighting. I can't say that this is important to corals or to any of the other livestock we keep. But it's cool, it's natural, and it's kind of fun. It's also what happens with nature, and it seems pretty logical to me that recreating the major aspects of nature as much as possible is a good thing. And none of it is possible with MH or any other light source.
Beware of misinformation. T5's are hardly a complete light source.
There is no difference between frags and colonies. They have the same requirements for light. As frags grow into colonies, they do shade themselves. That's a natural thing. But more on that later.
Nobody I've ever heard of uses two Radion XR30's for a 2'x2' space. That would be roughly equal to 700W of MH over the same space. A Radion XR30 is recommended for lighting 2'x2' for even the most demanding hobbyist situations. Also, I've heard of nobody running an XR30 at 100%. Typical is 60-70% of input power. And no, I have no stake in Radion, Ecotech, or any other lighting concern. I don't own Radions. Those who do almost universally love them. For many reasons. I don't know why anybody would argue with that.
One might argue, and rightly so, that LED lighting cannot provide a contiguous, full spectrum of light. This is true. MH can't either, but quality MH lamps are much closer to natural light in this . regard. Just the same, both laboratory and anecdotal testing has shown that high-quality LED sources can and do provide the wavelengths of light that are important to corals. Would you complain that your lights don't provide enough infrared? Of course not, because your corals don't care. They don't need it. But both LED and MH can give them what they do need, and equally well. If it concerns you that your coral colonies aren't colored up on their undersides, go take a dive. Of the four major reefs that I've gone diving in, the wild coral colonies were almost always white on their undersides. Sunlight doesn't shine upwards, and it often doesn't reflect well from the ocean floor. Consider this natural, because in most wild reefs, it is.
As for costs, I used to run 370 Watts of MH over the same 4' tank that I now light with two 135w AI Hydra 52 HD LED fixtures. My PAR readings are actually about 15% higher with the LED fixtures than they were with the MH. I also run the Hydras at about 60% of their maximum input power. That's roughly 160W. A difference of 210w. At 12 hours per day, that's 2.5kW less per day. 75.5kW per month. 920kW per year. That's a lot of wasted electric power. Plus the ridiculous cost of replacement lamps. Plus having to throw all of those old lamps into the landfill. My MH lights cost about the same as the AI LED fixtures I have now. Plus the cost of a controller to turn the MH lights on and off. Plus the cost of two ballasts that burned up. Plus the cost of cooling the aquarium and the house from all that heat they generated. To me, it's a no-brainer.
The idea that LED fixtures will only last 4-5 years (and I'm not sure how one can state that 'nobody keeps them longer than this') has more to do with evolving technology than with the efficacy of the fixtures. The useful life of top-quality LEDs is 50,000 hours, or roughly 11+ years. If they meet my needs now, they probably still will do so in 10 years. Just a few years ago I would not have been able to say that because the technology was changing so quickly. But the technology has improved tremendously since then.
I still have my MH lights. I have no idea why...
