how to make fish like buoyancy in 3d printed fish models?

[curious aardvark]

find a filament that floats and add weights.
different weight = different depth :-)

As to what plastic floats - no idea, never tested.

I am pretty sure, pla, petg and abs do not float.

https://bitfab.io/blog/3d-printing-materials-densities/

According to this, the only filament that might float is polypropylene. As it's the only one with a density less than water.

The other approach woud be to print in white pla and glue very small pieces of expanded polysterene to the fish.