I think it's aPlatygyra. At night do you see a lot of stingers coming out of it? That would be a tell tale sign about it. If you see a lot of stingers then you want to give it some room.
+1 platygyra....maze brain. when lights are off, turn off flow and squirt some thawed mysis juice over it and wait, the tentacles should come out. you may have to try several nights.