A new background using an improved panel system (4 foot x 2 foot size made of 8 x 8 inch panels with alignment biscuits) can be found here: https://www.thingiverse.com/thing:3564970

A "proof of concept" double sided aquarium background made of 8 ~14.5 cm panels for a finished product suitable for any 60cmx30cm tank. I recommend printing 3/4 of the panels for one layer, then gluing them together so you can get an accurate measurement and use your slicer to trim the final panel to fit in your specific tank.

Printing is easy, but assembly/alignment can be challenging; I've included the "background clips" file to help hold 2 panels in position while gluing.

I recommend using a CA glue such as Gorilla brand super glue and baking soda to fill gaps and quicken the glue's curing time (watch: https://www.youtube.com/watch?v=Z9qqEbPxx0k for details on the technique [not my video]).

Side 1: sharp detail canyon wall
Side 2: a rounded "negative" of side one, works as an earthen wall.

Future plans are to slice backgrounds into enough 8 inch panels to fit a maximum size of a 48 inch X 24 inch tank (or you can just print enough panels to fill smaller tanks) and too integrate alignment pins into each panel to make assembly easier.

Designed with 2mm walls and meant to be printed with 100% infill and no supports; thin walls in design make the object unsuitable for significant re-sizing (anything less than a 50% change should work).

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.