First off, you should have removed the dock at the first sign of this and replaced it with one that is non-edible ( I know there's some out there because I have one but can't remember the brand name). Gravel/small rock in the bottom of the tank is a no-no as well. Anything he's eating like this can become impacted (lodged inside his stomach/intestines) resulting in a very sick turtle that would require surgery (if he survives).
Usually the reason they are eating rocks/basking dock is that there's something nutritional missing in his diet. Try buying cuttlebone from the bird section of the pet store and offering to him ( remove the hard back by scraping it with a knife and then break into bite size/head size pieces and drop a few in the tank as needed). This should help.
Also, at their age, they should only be fed every other day "once a day" the amount of pellets that would fit inside their head if it were hollow. Veggies should be given on a daily basis though. What have you been feeding and how much? Overfeeding can lead to a lot of health problems down the road. Turtles are perfect beggars and can make you think they are starving... this is just their nature and we shouldn't give in since we have their best interests at heart as their owners.
Concentrate on getting them on a balanced diet and removing anything non-edible that they may be eating from their tank. That should help.
Posted: Thu Jun 22, 2006 9:55 pm zoo med turttle dock