Shredded tail fins?!?

When I came home from work last night, my black moor's tail fins were totally
shredded, and he's missing some scales on his side. I have absolutely no idea
what happened - there are two other gf in the tank but I've never seen them
attack one another. The substrate is large rounded natural stone. There's
nothing sharp in the tank. I have a "basket" over the end of the filter intake
to keep the fish from getting sucked into it.

My other two gf, a ryukin and an oranda, are just fine. But the moor's tail is
shredded, it's probably in a dozen long strips or more.

Water quality is great - just did a 30% water change a few days back, ammonia
and nitrites at 0 and nitrates are around 10ppm. PH is 7.2, which is what it
has always been. No changes recently in the tank, food, or filter.

So I have two questions. First, what in the world happened, and second, is
there anything I can do for the poor guy? He is having trouble swimming, but
is able to get around slowly and did eat this morning.




Amanda