I have four fairly large black ghost knives in a tank, all around 12" long. They've always been playful with each other, kind of chasing each other around, but never actually hurting each other. Today though, I noticed the two largest of them have been opening up their mouths wide, and trying to bite each other. The smaller of these two is usually the one who starts their fights. They haven't actually injured each other yet, but I'm afraid they are going to. should I be concerned about their fighting? Should I separate them? The only tank I could move one of them to... has a 20" black ghost knife in it.