Until the slightest parameter changes and nukes everything in you tank. A single sea apple getting picked on by an angel can take out a mature 300 gallon tank in a matter of hours.
I had an australian sea apple when I first started the hobby up until about 3 months ago (nearly three years). They're colourful and it's entertaining to watch them shove their tentacles into their mouth as they feed. As long as you have live phyto, active carbon, and stable params they're amazing to have in a tank.
Parameter changes won't cause the sea apple to nuke the tank. An angel picking on it, yes, probably. So you'll have to pick out the tank inhabitants well, and make sure they're 100% reef safe. Unless something eats the sea apple or it gets caught in a pump or wavemaker, it won't expel toxins and will instead shrink into nothingness. I wouldn't have recommended the sea apple if I hadn't seen that the OP was coming back to reefing, and wasn't new. This isn't a beginner animal, but one that you should plan your tank inhabitants around.
I heavily research anything before I purchase. Figured these were toxic similar to cucumbers, I’ll have to look into them more.
They are, but only under conditions under extreme stress. Any parameter changes that are dire enough to make a sea cucumber expel its organs would have to be bad enough to crash your tank anyway. I took on the challenge of caring for a sea apple when I was a new reefer, and while I did everything I could, information was limited and I didn't have live phyto, I only fed bottled dead phyto. Even still, it lived for three years, and only started to go downhill after I moved. The stress made the sea apple drop its feeding tentacles, and it was too slow in growing them back and slowly starved to death as it shrank into nothing. In hindsight, it would have been fine if I fed live phyto.
