If you can in the long run the best possible way to house sliders in your climate is to keep outdoors in a fenced pond. The shell curling has to do as well with to much protein and not enough fiber, with the size that they are now if they were wild they would be eating a mostly herbivorous diet. For your turts to accept their veggies it will likely need some training, but try to fload small bits of dark leafy greens in their tank for them to nible and then increase the amount as they get the idea.
