Hum there could be some shell rot in there all rigth. Your setup is ok, just make shure that artificial rock cant fall and hurt your turt, in general rocks take out too much space, a better island is a big chunk of cork and your turt can use the underneath as a hidding place. Is there any chance you could take your turt to see a exotic vet? I could descrive the treatment for you but its risky, shame if I were there I would treat that in a instant...