A classic UFO solid body disk designed for porcelain ceramic but also available in plastic materials. The central dome has a spherical hollow cavity with four open windows or portholes on top. The hollow cavity can be used to hold small objects such as paper clips. The bottom of the UFO has three rounded leg supports.
A version with a bottom hole can be found at the following
link:
https://www.shapeways.com/product/ZJ46J7KH4/ufo-solid-body-with-bottom-hole?optionId=59777678&key=d727e432ce0e6d4c46473a05a989d5b1
The safety of using 3D printed materials other than porcelain in aquariums with living creatures is debatable, so if you have doubts plan on using 3D printed parts in a decorative aquarium containing only plants, snails, and other robust creatures. If using in an aquarium it's probably best to go with a food safe porcelain material option rather than plastics or metals.