Hope this helps, right from live aquaria:
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.
Do not house these cucumbers in an aquarium that contains any fish that may pick on its tentacles. Generally, any fish that is prone to pick on feather dusters, will pick on the tentacles of the Indonesian Sea Apple. These fish include; Butterflyfish, Large Angels, and any species that is listed not safe with invertebrates. When feeding, it will extend its oral tentacles into the current to filter out food. As the food is trapped, it will then draw its entire tentacle into its mouth. In the aquarium, it will need to be fed a diet of phytoplankton - frozen, liquid, or dried - on a daily basis.
It is a really neat creature, that at one time I thought about getting one...but with no location near me getting them in I didn't want to try a toxic creature by ordering it and coming to me through the mail. I can nurse some difficult creatures from such a hard journey, but just didn't want to try it in my crowded tank.
If you like them, you might want to check out medusa worms. They are pretty crazy and have a neat relationship with emperor shrimp.