Just a turtle being a turtle...when they are young the "usually" have a great desire/like for meat....so I would perhaps back off a bit on the fish and try to encourage the pellets or maybe even transition to the freeze dried bug mix...crickets, mealworms and river shrimp....even a nightcrawler or two is a good switch (just be sure the nightcrawlers are not from soil high in animal waste)...
Crush the pellets into a powder and dump the fish in the powder before feeding. It should start him getting use to the Flavor. If be shocked if he still refused the powdered fish but let us know.
Then you may have to get surgical and open up the fish to insert whatever pellet dust you can fit in there. A small incision around the fishs cloaca and shove it in from there or the mouth of the fish.
It sounds like you have a turtle Einstein on your hands lol