Par is extremely important and corals will not photosynthesis to their full capacity without enough of it, especially sps corals. Lux is just a term for brightness to the human eye while par is “photosynthetic active radiation”. It only takes into consideration the wavelength that corals use for photosynthesis, basically 400nm - 700nm. The lowest par most sps will do well with is 150 and the highest is around 600. I am not saying it can't be done other ways but this is the sweet spot from all of the data I have read which is more then I would like to admit to.
Now I am not trying to start a debate because we all know we can run a reef in many different ways and light is only one important factor. Water quality, stable parameters, etc all make a big deal. But from a lighting perspective, PAR is very important and PAR drops of a cliff the deeper a tank goes. Let’s say the par was 700 at the surface of a 24 inch tank it would probably be 100 at the sand bed with a good t5 fixture and good bulbs. Through in a mediocre t5 fixture and average bulbs and do the math it won’t add up well.
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