Hey there
I've also gone through the same thing you did, having not cared for turtles properly the first time around.
I got two turtles about 4 years ago at a pet store and they were fine all that time, then suddenly they both got sick and died. It was horrible.. their little necks were swollen up, their eyes swollen shut...
When I took the 2nd one to the vet after finding the first one dead I was told that it was a vitamin deficiency. Nobody told me about the UV light and how to feed them... I was just told to feed them the turtle food they gave me and that they needed water to swim in.
I have 2 new turtles now and have done a -lot- of reading. I also plan to do things better this time around.