A basic aquarium cave. There are others like it, but this one is mine.

Designed very quickly in Blender starting with an icosahedron, add some subsurface subdivision, a noise displacement modifier, delete a couple of faces for openings, drag some of the points around a bit, add a 1mm thickness modifier, and done!

If you're like me you'll look at this model and see that it has a couple of "ambitious" overhang areas, but it printed out fine on my basic Anet A8 without any supports, so if your printer is better than that you shouldn't have any problems.

Printed with 0.2mm layers in "food safe" PLA, and it sinks to the bottom of the tank just fine (the walls are 1mm thick, so there's no infill area to trap air and make it buoyant).

I also printed with a 5mm brim to aid adhesion to the plate, but the brim also serves as a nice way to anchor the cave in place if you're at all worried about it floating - just leave the brim on and wiggle the cave into the substrate/sand of the aquarium and it'll stay where it is.

I printed in white (the kids wanted a fish "ice cave"), but I think it would have looked better in black or brown or something like that.

Print Settings

Printer Brand:

Anet

Printer: 

A8

Rafts:

No

Supports: 

No

Resolution:

.2

Infill: 

20%

Filament: 3DFillies PLA White

Notes:

The PLA is rated as "food safe", so (I guess) it should be OK in an aquarium too. I'm no expert though, so make your own informed choice.

Also, don't try to just scale this up - it most likely won't work.