I believe they need filter feeding constantly or will starve to death slowly.
This seems to be what has happened more often than wiping out a tank ASSUMING YOU LOOK AT IT DAILY !! as signs of stress and decay will show up before that happens.having carbon in the sump and a good sized skimmer should take care of most toxins assuming you remove it when you see problems.
This is also in the article NaH2O linked to under post #2 Sea Cucumbers part 2 Rob Toonen
" The dangerous reputation of this attractive sea cucumber is certainly based in reality, however, as I explained in my previous articles (Toonen 1998b, 2002, 2003), sea cucumbers have a variety of defenses that are generally tried before they resort to a general release of their toxic chemicals for defense. In general, it is only when a sea apple is extremely stressed that they will actually release the toxic chemicals that are capable of wiping out an aquarium, and many people have had one of these animals die in their tanks without any evidence of toxic release. Furthermore, there are many animals that we commonly keep in our tanks (in particular some sponges and zooanthids) that have much more potent chemical defenses than do sea apples. So, basically the main reason that sea apples are potentially dangerous to a reef aquarium has less to do with their chemical defenses than with the fact that they move around and are more likely to meet an unpleasant end in our reef tanks than an equally toxic sponge or soft coral. "
And yes another quote from the same article
" However, I do want to say something about the dire warnings that often appear on the web about ever adding a sea cucumber to an aquarium. While I will repeat that it certainly is possible for a sea apple to kill tankmates if sufficiently stressed, I have often included a sea apple in my own tanks without any problems. If proper precautions are taken to ensure that the sea apple is not damaged by a pump intake or overflow, chances are very slim that you’ll ever see any evidence of the toxicity of these animals. In my experience, reports of “cuke nukes” most often occur when the tank is small, runs an undersized (or no) skimmer, does not use or regularly replace carbon, or when the tank is not checked on a daily basis (such as when someone goes away for vacation and has a friend look after their tank)"
I am thinking of adding one to my tank .
Paul
