Which inorganic or organic value is most responsible for the darkening (even slight) of Sps corals?

[Anxur]

In your experience, which value in the tank increases the darkening (even slight) of Sps corals the most?

Obviously, we take for granted, perfect lighting, balanced and stable triad, movement etc.. So in a perfectly stable aquarium.

Thanks

 

[Randy Holmes-Farley]

The question doesn’t really specify how much of what, so folks will be answering based on attained levels that may be very different.