The sea apple may fit in there, but why?
Sea apples generally find one spot in a constant indirect flow and stay there. They are filter feeders that need some pretty heavy specific feedings. If they are not happy or well fed they will die and it is extremely toxic to your tank. They are in the cucumber family and cuke nuke is a real thing. If the apple feels threatened for some reason they will start to expel their inards as a last line of defense which is also toxic to everything else in the tank... they are an awesome animal and look really cool but I personally wouldn't put one in my sump since you generally don't give the sump as much attention as the dt and they need very specific care. If your confident you can care for one they are a really interesting addition to a reef. I had one for a few years in my seahorse tank since as an nps tank I was able to feed it easier etc but when it died due to a heater malfunction (few days earlier) it took out the tank without warning.