I'm pretty sure I didn't have move objects on as I was having trouble placing it on the wall I wanted to start with, turned out there was an invisible painting there, had to delete the wall and rebuild it then drag the tank out of inventory again and place it, after that the fish started appearing, had about 3 tanks worth upstairs and another tanks worth stuck through the front door.
I think my lot was just buggy, about 6 paintings I had tried to place vanished as soon as I placed them, they were still there as nothing else would go on the walls I just couldn't see them
Oddest thing was they would all go watch the floating fish and not the actual fish tank so it must be the fish not the object that calls sims to it, makes sense otherwise they'd stare at an empty aquarium if you had one.