You need an UV lamp to.we used to keep three terrapins when i lived at home. you are right they will need a fairly large tank, as they grow really fast. they also need to be able to get out of the water. you will need a heater, a filter and possibly a heat lamp (although i'm not sure about that, we didn't have one, but we lived in dubai, so it was always hot!)
you won't be able to put them in a tank with your fish if that is waht you are asking!!!
kat