If you have a clean glass bottom, the filter will remove any thing like that and the turtles will not be able to eat the gravel. If your turtle eats gravel, they can't digest it and you could have problems. Here is where fish and turtles need the same thing. Get a heater that is meant for your tank size. Also on the topic of temperature, get the little guys a basking lamp and a nice rock or ledge on which to use it.
Turtles get their body heat from their surroundings as they are cold blooded.
Giving them a nice warm basking lamp and rock on which to sit is a wonderful thing. Also, when the turtle can get completely out of the water and dry off, it allows the shell to harden. If the turtle can touch bottom and breathe at the same time, you need more water. What I recommend is that you get a submersible power filter that is designed for a fish tank 3-4 times the size of the one you have. For example, if you have a 10 gallon tank, get a filter designed for a fish tank that is 30-40 gallons in size.