Yes, the human tuna is what she's talking about. Just be sure that it is the kind packed in water not oil. I had to try this trick when I first got Koopa. If it works then eventually you will need to slowly wean him off the pellets soaked in tuna water to just plain pellets by adding less soaked pellets and more regular each day.
Another trick that worked well with my turtle was to feed the freeze dried shrimp that you buy at the pet store as a treat for your turtles. If he'll eat that, then one day add a pellet as you drop them in the tank. Daily increase the pellets and decrease the shrimp until he's eating pellets only.
Just remember that the amount of pellets he should be eating is the amount that would fit inside his head if it were hollow (estimation) once a day (every other day for older turtles - daily for hatchlings) with veggies offered daily.
Keep us informed and good luck.
