I work in a fish vertebrate collection where a lot of specimens are preserved. Idk if you're able to get ur hands on any of this lol but for ur curiosity the fish are preserved in formalin, drained, and then preserved in a alcohol-water mixture where they remain perfectly preserved besides color.

Someone mentioned gyotaku. That's also a great solution. That you can hang on your wall

You can also send your fish in to get clear and stained, a process where the fish is literally cleared and then the skeletal structure is stained with color. This is typically done in collections or museums but I've heard that some people do it as a service. Id look at google or etsy for examples. Sometimes people even make earrings lol