RES biting

Question
I have had a RES and what I believe is a western painted turtle for over 5 years now they use to live in a 60 gallon tank when they were growing up as they got bigger i always wanted a new tank. Once I moved out out bought a sweet 6ft by 18 inch tank they have lived in there for a good year problem free. I then added some gold fish still no problem one day I noticed that my one western painted turtle wasn't entering the water at all. I gave him a little persuasion into the water and noticed that the Res was biting at the Painted turtles tail feet anything it can bite to drive it out of the water and on to the rocks it will. Why is this happening now and not when they were grown and in the smaller tank years ago.  

Answer
Hello I am sorry that It took me so long to respond. I would recommend taking them out, changing their tank completly around. It can be a few things. possibly that they are mating, possibly there is a territory issue taking place. If you change the tank around, they will have to re-establish the tank territory and thus should assist in resolving the issue if in fact it is that,
please let me know how this works and if needed Ill assist more.