I have two RES,s that have lived togather for about 3 years and have got along fine. They lived in a 55 gal tank with about 6 in. of water in it, with one third of the tank had dry rock area to sun bathe. Today I dicided to clean the tank and fill it with water about 3/4 the way from the top so they could swim more, and to take the gravel out because they were eating it. Soon as I did this the bigger RES started to chase the smaller RES and bitting it. The large RES is about 6 in and the smaller one is 4 in. I took the smaller one out because I thought it was going to get kiiled.
Does any one have any idea why the larger RES would all of a sudden start attacking the smaller RES after 3 years? I am pretty sure that the larger RES is a female but I am not sure If the smaller one is male.
I don't want to separate these Turtles.
Thank for any help.

