Maybe Diablo's other tank was in a place where he could see other fish or his reflection? 10gs are way more reflective than 5gs. And it could be the filter. You could try putting a plastic cup in there to see if he'll nest under that.
The stupid plastic flower made everything look pyschadelic pink. But you can see the tip of Heartnet's anal fin has some blood on it. I think he might be fin biting.