Sounds to me like your little guy is seriously stressed or irritated. Sea apples can display a number of actions when bothered or stressed. Some will deflate themselves as small as the possibly can, and others will actually take on water, expanding their bodies to more than double their normal sizes in order to catch on a current and float away. In addition to the other signs of stress that I mentioned, the most drastic response a stressed cucumber can make is to expel its Cuvierian Tubules. The tubules of Cuvier are a series of long, spaghetti-like tubes leading off the hind-gut the cucumber equivalent of a colon at the base of the respiratory tree. The Cuvierian tubules are expelled by rupturing the hindgut, and this defensive ploy is almost always accompanied by the release of a soup of toxic chemicals that are likely to seriously impact, and potentially wipe out an enclosed tank. Although the effect of these chemicals are rarely lethal in the wild, more than 96% of predators exposed to a cucumber after having previously experienced the defensive chemicals remained far away from the cucumbers (Hamel & Mercier 2000). 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 youll 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 have never personally suffered a wipe out from any of these unfortunate events though. Catching the problem early, doing extensive water changes and increasing skimming and carbon use to remove the toxins as quickly as possible, has worked well. If any large animal, such as a sea cucumber, dies in an aquarium and is not immediately dealt with, it will drop water quality. It seems to be common that reports of tanks affected by sea cucumbers also include the sea cucumber itself among the casualties, and therefore, at least some of these cuke nukes reported are likely to be the result of insufficient attention to a decaying animal in the aquarium, because if the defensive toxins released by the apple, killed them as well - what would be the point?. If you could add some photo's that would be great, but I think what i mentioned above is the issue here. Good thing you seperated him soon, make sure he is by himself if you can that way no other animals suffer from his temper, LOL. What do you feed him also? And how often. Well, good luck, hope this helped some, keep us posted!
-Em
