How big is your tank? Sometimes when the tank is too small, turtles tend to get a bit territorial (kinda like having 3 kids in the backseat of a car for a loooong car ride!). They will push each other around & sometimes bite each other. This fighting can get worse as they get older/bigger & need more room.
The rule of thumb on this board is to allow 10 gallons per shell inch of turtle. So two one-inch turtles would need a 20 gallon tank, or one 4-inch turtle would need a minimum of a 40 gallon tank to feel like they aren't living in a closet.
It sounds like a lot (10 gallons per inch of turtle), but when you see a 9 inch turtle in a 90-gallon tank and how well she can spread out & not hit the wall when she moves around, trust me, it makes a LOT of sense.
