I think it is pretty sad to see as many of these as people are seeing in the LFS. See Apples can easily eat a bottle of phyto a month which can become quite a spendy creature to keep in your tank. They mostly eat phyto, 90%+. You also need to feed the live phyto and they are very specific as to what size particles they will consume. The dead phyto tends to clump up, becoming to large for the animal to eat.
They shouldn't be in a tank with corals as the stings can be stressful/fatal. They are normally stresses at the LFS. The pretty big ones at the LFS are normally the ones in the worst health as inflating is a stress responce. They can take over a year to slowly die in you tank so people don't notice that they are not providing for the creatures needs and think they are doing a good job with the animal, when they are not.
I don't think the risk of killing your tank is the problem with keeping them, it is the husbandry required. They are more work then soft corals that require feeding everyday. It is actually pretty rare to have them nuke a tank due to the use of their defences. More likely, they either die and pollute the tank like any large death, or they got sucked into a powerhead.
I would recommend you get some sun polyps or other non photosynthetic soft coral and see if you can get it to grow and reproduce before you try a sea apple. That would be a good indication that you can meet a sea apples needs and not risk your tank. If you do go for the sea apple, I would put it in a dedicated tank or a refugium. Plan on buying a bottle of DT's/Phyto Feast Live a month/or grow your own phyto.
i am in south bay. thanks for the input though. 
They're interesting, but they're a really large biomass if they check out in a closed tank. If you have one, run carbon, a lot of carbon, and be ruthless about netting it out if it shows any symptoms of imminent death.
) i will increase the amount of carbon. 
