I have had water turtles together in an aquarium for some time, but suddenly one of them is acting very aggressive. Why?
This can happen as turtles reach sexual maturity. As they mature and start seeking mates, males in particular may become agressive towards other males or may start to harass females. If the turtles are housed in a spacious outdoor habitat, this is seldom a problem as the turtles can get away from each other. When kept in an aquaurium there may be no way for the turtles to avoid contact with each other.
Aquariums are not good permanent homes for turtles. The aquarium will quickly be outgrown by most water turtles and, at least as important, they need access to direct unfiltered sunlight outside to remain healthy (In spite of information that you may find on the Internet and at pet stores). There simply is no available substitute for natural light.
Another problem with aquariums is that female turtles must have access to dirt in order to lay eggs. If denied this access, the turtle can become seriously ill and die.