I'm looking down into my fish tank, and the first thing I notice is that all the fish are swimming together in a clump. Then I notice it is because they are taking bites out of a dead one, who is missing his tail. His body ends in a straight line, and the back part is missing.

Then I notice that several other of them also have missing tails. Most of them are dead, and the ones that aren't are eating them. One is very sick and tailless and I think I should put him down. I need to clean them out, so the ones who aren't sick won't get sick from eating the other ones. Another one still has his tail, but he is brown with white spots.

Somebody is standing in my doorway, and I'm explaining, narrating all this. But hey, he's a doctor or something - can he look at these fish, what do they have?