Interesting Acro question

[Snappy]

Originally Posted by marie

When bleaching occurs there is no zooxanthella to protect so the pigment is lost and in low light conditions (or high nutrients) when a coral turns brown, what you are actually seeing is the colour of the zooxanthella itself trying to absorb as much light as possible.

The pastel corals that you see in some pics of zeovit tanks are actually partially bleached corals. The tank is so nutrient poor that it is having problems feeding the zooxanthella

Yes. Zooxanthellea are symbiotic algae that live in the gastrodermis and are photosynthetic. They produce carbohydrates for the coral to feed off of.
But in a nutrient poor environment like zeovit it is often a deficiancy of potassium that lightens the colours. The coral will expell some of the zooxanthellae it doesn't need in brighter light because their photosynthesis is more productive and the coral only keeps what it needs.
Clams also contain them and interestingly enough I've seen a blue clam go
brown when there isn't enough light. Anyway of course the coral does have a pigment and most zoox are brown but not all. Expelling the brown zooxa lets the coral get the nice colours but it still has them and they are not all brown. I think we are starting to split hairs now. For the most part we agree.