the main thing is, you absolutely can't get just one rat.
rats are highly social animals that need to be kept either in same sex pairs or groups.
keeping a rat alone is cruel and inhumane, and results in a wide range of health, behavioural and psychological problems.
no amount of human interaction can substitute for another rat for company.
it would be VERY SELFISH to ignore this fact, and get one rat. you'd only do that if you hate your pet and want it to be miserable, unhealthy and die young.
I don't know if your cage will be suitable. don't turtles need tanks?
rats absolutely can't be kept in tanks. rats need a large, multi level cage with bars. the MINIMUM size for a rat cage is four cubic feet, but bigger is better. rats NEED good ventilation because they have very sensitive respiratory systems. tanks make them very ill, and aren't good to make interesting for the rats. rats also need to climb.
you must never keep rats on pine or cedar shavings or sawdust. softwood releases phenols, which are corrosive to the respiratory tract. this leads to scarring, and greater susceptibility to infections.
most rats find carefresh too dusty.
personally I really like a paper pellet based cat litter for my rats.
rats are easily litter trained, so that makes this easier.
rats need a minimum of one hour out of the cage each day. this can't be in an excercise ball. they have poor ventilation, and rats find them distressing and disorientating. they need to free range, so they can explore and interact with the people and things around them.
if you choose to provide a wheel in their cage it must have no gaps to trap paws or tails, and be 12 inches across or larger. even then, most rats don't enjoy wheels.
rats need access to a good small animal vet, and it's very sensible to save money aside each week or month just in case they get sick. because if they do, you can't put it off until payday, or your day off or next week. you have to take them straight in at the next appointment. they can go downhill very quickly.