Originally published here on Thingiverse.

This is a buoy, or floater, designed to absorb the swells generated by the flow of air and water from the aquarium air pump. It's specially useful for floating plants like this water fern from genera Azolla, or duckweeds (Lemna). It works also as a floating platform for shrimps and amphipods I have in the aquarium. I added a voronoi part that is intended to slow down the water flow by "friction". It does not need any anchor, it does it by itself with the opposite surface flows.

I design this using SolidWorks, creating two parts, the buoy and the plummet. This last one was "voronised" using Voronator, then merging both parts. I didn't made any equation to design this, I just guessed; it still works fine. I do not recommend to scale it down, but if you do I suggest to scale the X and Y axis inversely with the Z axis. Say, if you scale XY by 75%, do it by 150% in Z. However, do not go lower than 75%, it would be too deformed and it probably wont float anyway. This is because if you scale down, the wall width remains the same, making it heavier. Remember that the PLA density is higher than water's. It's necessary to use no infill to asure buoyancy.