Red Ear Slider Aggression

Question
I have two Red Ear Sliders that are nine years old. I have one female and one male; the female is almost twice as big as the male despite them being the same age. Lately, I have noticed the female biting the male's neck without letting go unless I physically separate them. After separating them, the female would continuously attempt to do the same thing for a while.I take the two turtles out during the day in a separate place for having meals - the female does not attack the male when she's in this separate container. It only happens right after I return the two turtles into their tank. This has been going on for two weeks. We have not changed anything in the tank for a long time, so I do not know why she is acting this way. My turtles both have plentiful water (30 inch x 12 inch x 8 inch = 21 gallons of water) to swim in, and they have enough space to bask without being crowded. I hope you can help. Thanks so much.

Answer
RES females are almost always larger than males. That's basic information. Please research/. Males are not territorially aggressive like crocs or Iguanas, therefore they don't need to be larger, and the females are the ones who have to carry the eggs.  

Your turtles DO NOT have plentiful water. 21 gallons becomes too little once they are over about 2 years old. At nine years old, you should have them in a Koi pond! But there is the possibility that they have not been kept properly, and are quite undersized for their age. Do you have UVB for them?

You need to make an outdoor habitat for them, or get about a 175 gallon tank. The female may be stressed by the male, and RES turtles in captivity have killed each other this way by tearing the neck open.