Im hoping your are keeping them in a 6 by 2 foot tank correct? Their claws can indeed scrach the tank´s glass but nothing serious. One important data here is that this species can all of a sudden turn agressive with conspecifics, specialy small ones so if I were you I would separete them. Their powerfull jaws can make nasty wounds on the victims flippers and shell. Make shure to fix that. As for growth no, it doesant gets stunth by tank size, what stunths turts or fish for that mater is stress caused by a to small tank, agressive tank mates,etc, bad nutricion or intoxication by a poluted water. But a to small tank is a good way to start this process. So by all means keep upgrading your tanks to at least 1000liters by turt and preferably the doble when they reach adult hood. Of corse if you are well suckseded in keeping them, you have got yourself a life long project. This species is known to reach sexual maturity in the wild by their twenties and live a very long time

Keep very good water quality, perform very frequent water changes and give them a very good diet. Keep your eyes open for things like fungus.