I personally think it is just about preference and what's the latest trend.
I entered the hobby in 2006/7 and metal halides and T5s were what everyone had and corals, fish, owners, etc were happy.
Then at some point, people started using LEDs and saw that the intense blue color will show you the fluorescence of the corals, reds are much redder etc (more so than 20k MH I guess, and with LED supplementation vs an entire fixture).
Truly, I think that's the biggest selling point of LEDs. People like the visual. I do not believe that typical nano tank owners actually see long term substantial benefits cost wise.
Most tanks under 120g or so (a functional depth under 24" esp. with sand) do not need more than 2x250 watts of light and even then, only if they have SPS (and at that point I'm not sure if it is need vs want...for some particular point of measure as I've seen the same SPS growing pretty and very speedy under T5s, 250w and 400w all in the upper level of the tank) (I do not know why people with tank heights of 16" or less buy LEDs for the strength of the light, especially if their tank has very few to no SPS. What is the point of paying $700 for a fixture that when you are only going to use 10-20% of it's power??).
So, for most nano tanks, I do not think electricity bill should really be that much of a game changer. When I ran 2x250w (run ~6 hrs per day) plus all of my other reef equipment, my bill wasn't over $30 a month if that (due to the tank).
The other aspect of the light issue people talk about is the cost per bulb/unit and electricity bill. Since I constantly see people selling their LEDs I doubt most people get much out of the supposed 5 year life span of an LED. The constant upgrading, changing out of leds for a different color, etc kind of negates the statement that LEDs will be made up in cost over the fixtures life time. LEDs cost 500-700+ for just one unit of the primo fixtures with options you dont NEED (thunderstorms, clouds). Selling that within 2-3 years, makes it more like a pay up front lease kind of deal and depending on the brand, it may or may not have a decent resale value (and of course, what is the next fixture type of interest). Also...my MH bulbs cost maybe 50-60$ each, that's $120 or less per year and is relly the only thing I'd have to replace. On a retro fixture, after 5 years, it still costs less than one of the primo LED fixtures.
Then there is the heat. People without canopies should have less of an issue, and should probably have a home AC if their ambient temp indoors gets above 80 degrees for their own health concerns. But even running my 2x250w in a canopy without an ac 9mos out of the year my ambient temp and tank temp never goes above 80 degrees.
Fans are a good option for heat. AND fans are still used my many people with LEDs. So, it's not like the expense /clutter of fans (or chillers) are being taken away 100%.
So, in my mind, the LED purchase cost over equivalent lighting seems to be a more cost/heat/etc efficient choice for tanks over 150g medium-heavy SPS domination, who intend to keep their purchased LED fixtures AS IS for at least 5 years (i mention 5 because websites and reefers always mention that in 5 years you'll make up the purchase price for what you would have paid in electricity and overall cost).
Anyway, now we have lower cost options on LEDs that work well enough and make the tank look pretty without trying/having good husbandry. So, most of the cost efficiency argument is unnecessary right?
For instance, I will get the D120 value LEDs for $130-179 used or new (2 units) as a back up for if anything happens to my t5 fixture. Why? Because it's inexpensive, it will grow coral, i do not need thunderstorms, I do not need to simulate the passage of the sun, and I can change how blue it is easier on this led fixture than my t5 where i have to take down the entire unit to change bulbs. It's also because of the hype. Running deeper blues look nice. But i'd only run it like that for an hour or two. I prefer a 12-14k look.
