If we look at the compensation points on many corals, we see it is well below 100 micromole/m2/sec, hence my general recommendation (and Sanjay Joshi's as well) to provide an intensity of 100 micromole/m2/sec. Of course, spectrum comes into play, but this intensity using many lighting sources will promote success.IMHO
Dirty aka Unclear Water can be nutrient deficient as in sand/silt or nutrient rich as in river or coastal development runoff. Unclear nutrient rich water in our captive reefs is a recipe for nuisance algae that more often than not may not be kept at bay with cleanup critters or a huge skimmer. Strong lighting will only make this worse.
Some of the waters of the Kona Coast offer a huge amount of clean nutrients to corals from micro organisms. Sadly its very difficult to recreate this environment in our homes... The Monterey Bay Aquarium (one of the best in the world) has the luxury to feed its corals this nutrient rich (loaded with zooplankton) water to its corals with little algae promoting nutrients... Note that both Monterey and Kona have a lot of very deep water and currents....
For my Personal Reef, I know that I cannot provide this abundance to my corals so I choose a very clean water with a lighting intensity that has worked in the past... 400 watt MH Equivalent or 200+ PAR on the sand bed.
A lot of very popular LEDs only offer 100 PAR on the sand bed at best.
Yes others have seen growth with less but corals are very adaptable and if the light is not strong enough they rely on feeding.
For those of you that will accept a poor or adequate spectrum, know that most Chinese Imports have a variable spectrum. One box will provide plenty of the needed 420-460nm another will be very weak in the 410-430nm... Others will have a lot in the > than 500nm which nuisance algae use. They simply do not bin their LEDs...
T5 lighting is good but the Peak PAR on an 8 tube ATI after 6-9 months drops quicker than Harvey Weinstein's pants during a hollywood leading lady casting.... Everytime corals are subjected to change its added stress, especially with lights...
Hope this helps
Bill