Our zebra danios have gotten scary. Where once they’d dash about madly at the top of the tank waiting for the flakes of food, now they wake slowly from sleep, huddled together, then in a three fish column begin moving slowly (not their usual frantic dash) into the plants, moving around them, seeking meat. The flakes of fish food float down all around them but they pay no attention. They keep prowling, methodically, maybe an inch or two from the bottom of the tank. I’ve come to suspect that this was how they killed the other fish, by catching them before they were completely awake there amid the plants. I can only imagine that all three would rush in, striking, chomping, killing. In the wild they eat insects and crustaceans and worms, so they are hunters, yet in the thirty some years we’ve been stocking our aquariums with them I have never witnessed them do anything more than grab flakes of fish food drifting by. I have certainly never seen this sort of apparently coordinated behavior. It seems that almost every vertebrate has within it the predatory behavior. We are all hunters. Hell, it was predation that drove evolution itself, the whole Cambrian Explosion with all its crazy speciation was the result of the ever evolving contest between predator and prey. And here, somehow, in our little aquarium, something turned these little inch and half long fish from eaters of fish food to eaters of fish, eaters of even their own kind (as there were five of them just two weeks ago). All was peaceful until the clown loach died. That loach, though never deliberately bothering any of the other fish, was at seven inches long to them like a whale shark is to us. It ruled the floor, digging up snails. The danios stayed up several inches in the tank, away from its sudden movements. But then the loach, one day two or three weeks ago, was dead. Old age. I noticed the next morning that the danios were down zipping around at the bottom of the tank. The neon tetras calmly minded their own business, the two glass catfish scooted about. Everyone, danios included, got very excited at feeding time, like they always did. Everyone swam around excitedly, grabbing bits of tetra min flakes floating by.
I’m not sure when exactly the danios turned into killers. Within two weeks I realized that all the fish were gone but these three zebra danios. Alone in the tank, they chased each other madly about, zipping one way, then another. I was mystified. Where had all the other fish gone? I did some research, and found desperate pleas on aquarium websites. “Help, my zebra danios are eating each other!!!!” or “My zebra danios are killing my other fish!!!” I read in shock just how murderous the little beasts can be. No one seemed to know why, but there was usually a dominant fish that sets it off. A handy bit of evolution, that, where some members of the species will suddenly go rogue, turn alpha, and eat everything piscene in sight. Obviously there is a genetic advantage in there somehow. Perhaps a surge in zebra danio testosterone. But I have no idea. Looking at the tank again, one of the danios is swimming like a lunatic now, frenzied. The other two have ducked behind the leaves. Perhaps there is murder afoot.