How to input objects into the bottom of a water tank.

[Danger]

I am; unfortunately Dave is no longer a member of the PF community.

 

[Simon Bridge]

Well, the post #20 design, with a return mechanism, would be one way to spoof a buoyancy pmm.
These things can be quite neat as art - a water-feature?

Dave's was a simpler idea which also addressed, indirectly, the main begged question: why not just push the balls in? Why gravity feed? (The grav feed is a common requirement for the buoyancy pmm designs.) Your's has the needed distractions.