Small wood textured pleco (or any fish, shrimp or reptiles) cave. 15.3cm X 4.6cm X 3.5cm.
Designed with 2mm walls and meant to be printed with 100% infill and no supports; thin walls in design make it unsuitable for significant re-sizing.
Print as oriented too avoid the need for supports. Bottom of cave left open to save on filament
PLA and PETG are both denser than freshwater so 100% infill will naturally sink to the bottom of the tank.
On safety and longevity:
Both PLA and PETG plastics are generally considered food safe but some of the dyes and additives might not be, so I use "natural" colour to be safe.
The decorations are painted with non toxic acrylics and coated with plasti-dip (also generally considered aquarium safe) to seal the paint.
I recommend printing in PETG long term since PLA is biodegradable and the presence of heat and water will cause it to break down (though from all accounts I've read, it will still last for over a year).
Disclaimer: I'm a hobbyist, and everything I've stated is based on personal experience and google search results; I recommend doing your own research and consulting professionals whenever possible.
Any PLA or PETG