I voted watermill. My very first aquarium as a child had one, I like the movements and it seems to have a purpose. I dig the castle bit but most don't move, they just sit there looking regal. Unmoving, but regal. Guess I need the justification. Course, who could REALLY justify an underwater watermill?? ( more beyond beyond stuff) I like the bubbles coming off the mill and the spin of the wheel.
But then again, that's just me.
gday
It's not having what you want, but wanting what you got. SC