I think that the golden/brown zooxanthellae are responsible for the darker pigments. This speaks to the paradox of contemporary methods of keeping stoney corals IMO.
It has been our goal for a long time to keep tank water low in dissoved nutrients like on natural coral reefs. Corals have adapted to survive in this environment with lots of mouths for eating, and the zooxanthellae recycle the coral poo, turning it into carbohydrates/sugar that the coral then makes mucus with. In an aquarium though, its not always the same. If there are excess dissolved nutrients in the water column, the zooxanthellae get to eat, but the coral eats less. That can throw the symbiosis out of balance, causing slower growth, and poor health. However, nowadays folks are keeping dissoved nutrients much lower than before, but may or may not be providing enough nutrients in the form of prey capture. If there is less food for the coral, then there is in turn less coral poo that feed the zooxanthellae. They in turn die and/or are expelled.
I have never seen a real reef, but from the pictures and moves that I have seen, there has not been any 'pastel' colored corals, and likewise there has always been crap (aggregates/detritus) floating all over the place. That leads me to believe that 'light/pastel' corals are in fact malnourished. Now don't get me wrong, zeo tanks, and rainbow colored 'sps' reef tanks can be quite a sight to behold, and it sure does seem like the corals can adapt and grow quite well in this environment, so I can sure see why this is the goal of many hobbyists. But that does not nessesarly mean that the coral is in a 'natural' state, or that the coral is 'healthy'.
When the subject of malnourished/bleached corals comes up in this forum the conventional wisdom is
Add a fish or 5 and increase the feedings to once a day at least.
But I am not convinced that this is the best answer. I have no doubt that it works, but that does not mean that its the best method.
First of all, more fish will result in more dissolved nitrogen (ammonia) in the water column. Often the tanks with these issues are already lacking sand, and/or algae, and, we are using less live rock than we did before. It seems to me that these are best lines of defence in combating this unwanted byproduct. In the past, I have suggested the intermentant use of the protein skimmer, but that idea was dissmissed because of concern for ammonia, and oxygen. But that for some reason dosn't apply for adding fish.
Second, digestion comes at an energy cost to the coral, so one might perhaps consider the nutrient profile of the food that the coral is eating. Take flakefood for example, who knows whats in it to begin with, then its dried, stored for god knows how long, fed to the fish, digested, and then the coral eats it. Better than nothing, but why not feed gut loaded zooplanton to the fish? Why not just feed it to the coral? If you like to use seafood, why use the fish to make it small enough for the coral to eat? Wouldn't a kitchen appliance work just as well?
Anton
