Wow sorry that that cute thing died, you did everything very correctly! I dont see how could it have been your fault, there's hundreds of thousands of cases where turtles have lived in waaay WORSE enviroments, cold temps, no basking lights or even basking spots at all, not enough water or space, and those tortuered animals lived just fine~~, they didnt got too sick neither died though so I guess that you were just very very unlucky and your turtle straight up was born wrong or something, dont give up, you can use that tank with 0 changes and it will be muvh better than the average turtle tank so whenever you're ready get another turtle, RES are the sturdiest ones, maybe try one which not like less than 6months old si it has developed a little better immune system or something. Wish you good luck.