I’m starting a new shrimp tank soon and I want to fill it with hollow tetrapods, commonly used for reenforcing shorelines. Their stable, interlocking shape combined with oblique surfaces for dispersing wave energy is idea. I just think they look cool, and think a carpet of them in a shrimp tank is both very aquatic (I see these all over shorelines I love).

I modeled a parametric 3D model of them using OpenSCAD, a programming language for creating 3D objects, and published it under CC-BY licensing on Thingiverse. Check it out!

My printer is churning some out as I type.

Also, TIL that the angle between all legs of a tetrapod is acos(-1/3).