IN my experience SPS need some nutrients in the water. Most ULNS have pastel washed out colors, probably cause light penetrates the water better, therefor less zooxanthellae, algae or "color", is produced by coral due to increased efficiency.
Even though you will hear SPS thrive in nutrient poor environments its kinda a double edge sword. Of course the ocean will appear nutrient poor, its a huge ocean. Its as if you have a 5000 gallon system, it will be a long time before nutrients build up to a problematic level. Same thing in the ocean, its the biggest aquarium ever, think about how much food you would have to put in to contaminate the system...Doesn't mean there is a lack of nutrients there though, its a balance. Think about the all the diverse life forms on the reef, living, dying, eating, expelling waste, reproducing, competing, 24/7. Lots of nutrients in the water to sustain life, its just the water volume is soooooo large its not at toxic levels.
So I guess what Im trying to say is when we try to duplicate the ocean in out glass boxes its all about balance and stability. Don't want to over strip your system and leave it deprived of nutrients, also don't want nasty yellow water, algae growth and corals that die.
I have had SPS with intense deep dark coloration and a higher than normal nitrate level, growth will slow though. Im all for stripping systems of phosphate, but i think some nitrate is not the worst thing. Kh, Ca, Ph, Salinity, dissolved oxygen all very important, when these are stable then worry about other elements such as potassium, iodine...
SPS corals, mainly Acropora are voracious predators. The animal has 90% of its body dedicated to capturing prey. How do you eat in a ULN system???
Again this is only my experience
