What brand are you feeding them and have you tried to feed them any other brands?
My turtle ate reptomin baby and then reptomin from the start, but some turtles are more finicky and there might be something in the brand that you use that they just don't like.
It's good that they're eating veggies, be sure and give them a good variety (look at the feeding stickys for feeding tips). The fact that the tuna couldn't bribe them makes me think that they really have something against those particular pellets, I think the next step would be to try a different brand.
If it weren't for the minnows, I'd say you had vegetarian turtles
