In addition to UV lighting, they also need a very warm basking spot out of the water in the tank. Shellrot is also a huge concern and can be fatal. Without the ability to thermo-regulate out of the water in an area 85-90 degrees, which also allows them to dry out, they cannot digest food properly or heal from minor illnesses. They can also suffer from respiratory infections as well.
Avoid the caresheets from pet stores, there are some really great forums online that can offer a lot of advice.
before you get another turtle i would google the kind and get a care sheet..in fact read a lot of care sheets...theres so many things that could have gone worng with your turtle...and turtles are different..whats good for one is not always good for another...but if you think it was uv and only uv i just don't think so..theres more going on there..when you say things like proper diet or apparently needed sun..thats a red flag telling me you need to research ...good luck ...billy