This doesn’t relate to how to fatten them up but might give another view.
I caught a 16 3/4 inch crappie and wanted to get it mounted and weighed for a big fish award also. So i had to take it to and authorized (certified) scale. I was feeling bad that I kept and took this fish out of the pond
just so I could put it on my wall.
After talking with the person where I got it weighed they said the fish had hit its prime and was never going to get bigger and only smaller. It weighed in a 2 pounds and I had it mounted.
If all of the Crappies and other fish are skinny I agree to clean a bunch of fish out. My thought would be leave the larger fish of any species alone and clean up medium to small fish first an adjust year by year.
But if its just a few maybe they are just older fish and out of there prime.