You got me curious so I did a quick search on the boards and google to see what I could find out about them - yes they are toxic, and will release a toxin that poisons everything in the tank if they die.
It sounds like some people have been able to keep them pretty successfully, but lots others have not, and paid the price for it.
I found this on another site and it seems to agree with the other stuff I read in the last few minutes
"It prefers an established reef aquarium with room to move about, and large amounts of live rock from which it receives a majority of its food. Once it finds a favorable location, usually in a high current area, the Sea Apple will generally stay there. Protect all intakes on pumps and power heads, to eliminate any possibility of being sucked in. It is very sensitive to high levels of copper-based medications. Sudden changes in pH or salinity can cause it to shed its oral tentacles."
IMO I wouldn't want to put something in my tank that has the level of care it sounds like these have especially considering the potential of releasing a poison that kills everything else, if it should die. I think thats one critter best left in the ocean, as cool as they are.
