You basically have two types of cucumbers available in the hobby, sand-sifters, and, plancktonic feeders. The sand-sifters, although rather homely, are excellant for cleaning the sand bed as they ingest sand and digest off the bacteria and algae and then 'return' it to the tank as clean white sand. The planton feeders are generally the more colorful, but, can be more demanding by requiring frequent feedings of plantonic type foods which they trap in tenticals they wave in the currents. Both type are very interesting, and, can be quite hardy. All do have toxic mesenterial filiments in their insides that they may expel as last resort form of defense if they are picked on by other tank mates, or even if severly stressed by tank conditions or shipping, so, it is important not let water conditions deteriorate, and, not to house with fish or inverts that will pick on them or try to eat thier tube feet or tenticals.
As to toxicity, it varies, but, the brightly colored types generally are more toxic than the drabber colored sand-sifters. Indeed some, like the gorgeously colored sea apples, do have the ability to nuke a whole tank...itself included if it is forced to expell its innerds. The sand sifters are less likely to cause any problems. I've personally only kept the black or yellow Holothuria sp. sand-sifting cucumbers and never had a problem. Even when one died and decomposed in the back of my reef nothing was affected by it, so, I personnally consider them pretty safe.
