Hon, I appologize if I'm coming off very blunt. I realise you're concerned but this is why I said do the search, most of the answers to your questions are all here already.
I'd get the gravel out of the tank and put some smooth river rocks in that are larger than your turtle's head. Better for your turt (no temptation to eat them and possibly become impacted) and easier to clean the tank.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-