Hi, I'm really worried about my turtles, I have 2 male Yellow Belly Slider turtles. They are both about 6+ inch shells, but one is stronger than the other. They are in a 60-gal tank I think. Well it's 3-ft long, 1-ft across and 1.5ft in height. They often put their hand up to each other and vibrate their nails. But 3 days ago I saw the big one bite the other one on the neck really hard, and today I saw him bite him again and this time he would not let go, he bit him on the underside of his neck. I took them out of the water but he still would not let go, what can I do?