It could be aggression if you've noticed them seeming to fight with each other or bite at each other. If they haven't it may just be lack of a good environment, like Jen said.
The one thing that makes me think aggression is the size of tank you have. The minimum size tank for a turtle is 10gallons per inch of turtle. Therefore to start off with you needed at least a forty gallon if your turtles were 2in each. And then you needed the water level as high as you can fill it without giving them room to escape. If they were forced to live it such a small space it could cause aggression (territorial disputes) along with possible illness.
You said that your UVB light kept the water temp at 78-80. Well, a UVB light does not give off any heat whatsoever. Sounds like you just have a UVA light. They do need both kinds. Water temp should be a constant 75 for older turtles and 78 for hatchlings with the basking temp about 90 or at least 10 degrees more than the water temp.
Have you read any on the main site?:
www.redearslider.com
There's a lot of great info on there to help you and your turtles....
Sorry I couldn't be help more....
And welcome to the forum...
