hi could I get some advice, I had 2 red belly piranhas but down to one now. I noticed when they were together they would chase each other and bite each other at times, it's like even though they shoalso together it's like they don't trust each other. I don't think I want to add any more piranhas incase they don't get on. but is it ok to keep a piranha on its own? I might add few more fake plants as he might be a little scared now. thanks.