They get spooked prety easy, they might be getting scared in the night and thrashing around. Sometimes it doesn't need to be a sharp object to cut them. When they lose it, they swim into things full tilt and a blunt object can cause a cut when they hit it so hard.
Something you can try is rearranging the tank to try and get them messed up sort to speak.
What worked for me really well was I have alot of drift wood and plants in my oscar tank. And when they were being really aggressive for a few months, I would position the wood and plants more in the middle of the tank so that you couldn't see the other side.
So they both had their spots where they could chill out, but they could still swim freely through the drift wood and plants to mill around.
Turning off the lights helps too, and lowing the water temp since it kind of makes them a little more docile.
But yeah, a bigger tank will be a good idea. Mine are in a custom 80g tank that is 5 feet long, but only 15" deep and at 10-11" they are looking mighty tiny in there. 120 is in the works
