Hmm. Don't know if a fish skeleton would dissolve or something because of the cartilage or delicate structure, but if you aren't squeamish you could bury it somewhere safe in the dirt for a few months in a wrapping that will let the buggies in there. Once the meat is gone you could bleach it and piece it back together. You do it with 12% hair bleaching peroxide from the salon supply store, I think you are supposed to use it before you dye your hair. That's if you just want the bones lol.
I cleaned up a dead possum once.
It was runned over in my neighborhood and animal control never showed up. I thought the teeth looked pretty badass (was like a full sized male or something, HUGE canines) so my friend and I got a big trash bag, stuffed it in there, and then buried the poor dude in my dad's woodpile for the bugs to eat, and to keep it safe from cats or birds.
The skeleton was pretty much all the way clean except for some skin and tendons about 6 months later, I think I waited all through summer and fall. It then was soaked in a bucket for about 2 weeks (freaked my mom out and really didn't smell very pleasant haha) so the rest of the meat became easier to get out of it, and then I razor scraped it gently and put it in a peroxide bath for about 4 days. It was similar to how they do it for museums. I had a link saved for someone who did a similar thing with a dead fox they found but no idea where it went. You should be able to find it on youtube I would guess. Or find a taxidermy person. Or the whole formaldehyde thing, if you know a biology teacher they may be able to order some for you! One of mine in high school had a ton of critters he preserved.
I'll get a pic of it next time I'm at my parents, I made a display case for all the bones I saved. I have most of it. I think I only lost the tail and a few sinus bones and a few tiny tiny teeth. The tail was mummified and I couldn't clean it so I tossed it and I mistook the sinus bones for crap pieces. The skull was pretty fractured up on top after working with it so I think it got clipped by a car and died quickly, RIP fluffy possum.
Science is fhun. Ha.