Tropical Fish Hobbyist - October 2010

this? The coral always re-expands afterward, and it seems perfectly healthy otherwise. The fishes in my tank actually seem to like the brown stuff, so whenever the coral does this, they swim right up and start eating. Charles Warner via email

James Fatherree

For our purposes, the terms alkalinity and buffering capacity are essentially interchangeable. Alkalinity is the water’s ability to resist a change in pH. This ability is imparted by so-called buffering agents, or buffers, such as carbonates and bicarbonates, dissolved in the water, which help neutralize acids that are introduced (usually in the form of fish waste, decomposing food, etc.). The more buffering agents that are present in the water—i.e., the A

The brown stringy substance you describe is likely zooxanthellae, and this behavior is usually nothing to be concerned about.

It’s perfectly normal for corals to periodically expel zooxanthellae in order to regulate the population of these symbiotic algae in their tissues. Oftentimes this will happen in response to some changing environmental factor, such as the gradual loss of light intensity that can occur as aquarium lamps age, or the sudden increase in intensity that occurs right after they are replaced.

I should point out that if environmental changes are too dramatic—for instance, if a specimen is exposed to precipitously high water temperatures for a long time, or there is a sudden, dramatic change in light intensity—it can respond by expelling all of its zooxanthellae and take on a much lighter appearance. This process, known as coral bleaching, usually leads to the demise of the specimen. Based on your description, however, I’m fairly confident that your coral is simply adjusting the population of its resident zooxanthellae and not undergoing a bleaching event.

A
Q
Alkalinity
and Buffering
Capacity

I’m a brand-new hobbyist, and I

just have a quick question for you about some terminology I keep coming across in aquarium books and on aquarium-related websites. Can you clarify the difference between water’s alkalinity and its buffering capacity?

Josh Norton via email

Red brain coral; corals can periodically expel excess zooxanthellae (symbiotic algae).

when you’re keeping sensitive reef invertebrates.

If I had to choose between Dendrochirus biocellatus and D. zebra, I would probably pick D. zebra. This species is generally more hardy and adaptable than D. biocellatus and has a much better track record of learning to feed in captivity.

Brain Coral
Spewing
Brown Strands

Every once in a while, my open brain coral will suddenly contract its tissues and eject a stringy, dark-brown substance from its mouths. Should I be worried about