So when our ancient loach died, half a foot long, some piscene balance of terror was upset, apparently, and the zebra danios went mad and ate the neon tetras and glass catfish and two of their own kind. There are three fish left in the tank, all of them zebra danios, all chasing each other madly throughout the underwater plants like crazy little cannibals. Soon, there will be two. Then one. At that point I will flush the murderous little survivor down the toilet and restock our tank with more sociable fish. We have had this tank for over twenty years and the two preceding it now for another ten or fifteen, a continuous aquarium for nearly all our thirty seven years of marriage. Once in the eighties we inadvertently bought a fish infected with ich (Ichthyophthirius multifiliis) and all the fish died but one platy, and for a year we had just the one platy as it slowly recovered, till we began adding other fish, cheap little neon tetras I’m sure, a couple at a time. No one got sick and eventually the tank was full of life again, and the platy lived several healthy years, even graduating to the next larger tank. That was probably thirty years ago. All has been fishfully peaceful since, the occupants remarkably long lived, until now. I watch the zebra danios in their mad dashes. They never stop. Even by a danio’s hyperkinetic standards this is frenzied. I had noticed that the nightly feeding wasn’t getting them as excited as it did just a couple weeks ago. But why would it? They crave flesh.