I am by no means an expert on anything! I have three sea apples (still do) for many years)
Here is what I have learned/recommend if you want to keep them healthy:
Sea apples will not tolerate water temperate above 76 degrees for more than a few hours (a chiller is a must)
I keep my tank between 72 and 75 degrees.
You must feed them phytoplankton (good quality) every day After the daylight light are off. I have not found Marine Snow to be of food value to sea apples.
Sea apples feed at dawn/dusk and night, if you see them feeding during daylight hours, they are not getting enough food (occasional feeding, or feeding arms out for a short time in daylight is OK)
Be wary of “reef safe” wrasses. They re not always safe for invertebrates!!
Nitrates should never exceed 15 ppm
It is a myth that you cannot move them. Every once in a while mine decide to “walk” to some ridiculous place in the tank. Here is what I have found to work: Turn off the filters and pumps. Carefully pull them away from the problem area. They attach themselves with a lot of power, so do not be alarmed if it seems like you are ripping them apart! Remove from the “bottom” first, never pull the sides or top). Place them in an area where they get good moderate water flow and gently hold them against the sand or rock where you want them. They will attach quickly. Once they attach, turn on the pumps and filter
Power heads inside the tank, must have sponge filter around the intakes.
Do not be alarmed if they start to take on weird shapes, they are fine as long as they are not folding in on themselves.
Sea apples should feel firm and plump to the touch. If they start to flatten, or feel “mushy” they are in trouble and should be removed from the take immediately!
All in all, despite the effort, I love them, just got an Australian sea apple that is so beautiful! I love them and find them worth the effort
FYI, I have a 28 gallon nanocube!