Supplement to “Relatively Reef-Safe” by Bob Fenner, Marine Fish and Reef USA magazine, 2012, Vol. 14.
What generally hobbyists call “corals” are part of a phylum scientists label as Cnidaria (nigh-dare-ee-ya), aka “the stinging-celled animals.” Not all of these are very stingy or sticky (agglutinant), but there are several that are popular with hobbyists and can be trouble to their tankmates and even their owners! What’s more, there are some very nasty chemically warring groups of cnidarians that you should be aware of; these should be placed last (perhaps not at all in some cases) after your other “corals” have been placed.
The list of toxic polypoid life is more inclusive than exclusive (i.e., almost all are to some degree stinging or chemically warring or both to some extent). Knowing which ones are more trouble than others is important in devising a livestock plan and systematically stocking your reef.
Zoanthids, some mushrooms, stony corals of the genera Euphyllia, Galaxea and Catalaphyllia are among the worst offenders though, and once again, there is a scale of offensiveness to the whole phylum. This being stated, these organisms can be kept together given a few provisos. One that’s been mentioned is to place the worst groups last or after the more peaceful have become established.
Second in importance is spacing their colonies. Oculinids (galaxy corals in this instance) can “reach out and sting someone” a good foot away. You need to provide “demilitarized zones” around such stingers.
The third idea is to keep the system optimized and stable. Anything less stirs up physical and chemical warfare. As part of the general health of your system, look to redox (oxidation-reduction) by keeping your skimmer in good operation and the periodic use of chemical filtrants to ward off troubles.
Be observant and proactive! There are almost always signs of trouble – organisms not opening or not feeding – that portend allelopathogenic trouble.