Sea Apples are more toxic then most cucumbers. But, it depends on what type of cucumber you have. I'm not sure what type you have.
From this article:
Aquarium Invertebrates: Sea Cucumbers - Part II
So Which Cucumbers Are Safe For A Reef Tank?
Well, to recap what we’ve learned so far, different species tend to have different specific sets of chemical defenses, and those chemicals tend to have different effects on different potential predators. So, obviously it is virtually impossible to make any real generalizations about the specific toxicity of a given species of sea cucumber or how their chemical defense is likely to impact your reef tank. In general, it is probably reasonable to say that sand-feeding turd cukes are less toxic than the colorful filter feeders, or more specifically,
the concentrations and variety of toxic chemicals found in the Aspidochirotiacea – the group that contains the sand-feeding cucumbers – is generally lower than those of the Dendrochirotacea – the group that contains most of the colorful suspension feeding cucumbers (Paul 1992; Pawlik 1993). However, there are plenty of exceptions to that generalization, and the fact that there is a strong behavioral component makes it impossible to predict the effect of any given cucumber in any given tank when something particular happens. Ultimately, the real question that most likely matters to you is not which sea cucumbers are more or less toxic, but rather how much of a risk is some particular sea cucumber to the rest of the critters in your tank.
Other articles of interest:
Good read:
Sea Cucumbers: Nature's Calcium Reactor?
Aquarium Invertebrates: Sea Apples
Hopefully this is helpful for decision making, but definitely informative. Can you take it back and get a smaller one?
