I had a little aquarium on my desk before we moved. Now that we have space I'm setting up a much larger tank but I haven't transfered over my fish yet. So right now my little aquarium is sitting on a dresser in front of a window. It's a west-facing window, so in the evening the sunlight comes pouring in the side of the tank. I looked in my little aquarium a couple of evenings ago and my cardinal tetras were swimming on their sides. Something had to be wrong! A terrible tetra plague? Inner ear infections for all? I immediately called in my fish biologist spouse. He took one look at the tank and laughed. It turns out my fish were just confused.
As I've mentioned in one of my other blog posts, cardinal tetras are small fish with vivid iridescent blue and deep scarlet stripes. Cardinal tetras come from shallow tributaries of the Negro and Orinoco Rivers in South America. Their habitat changes as wet and dry seasons cycle each year, and they move from flooded forest areas to crowded streams when the water is low. The middle Negro is the primary fishing area for these fish, where about 20 million of them are captured annually. About 90 % of fresh water aquarium fish are bred in captivity, but some, like cardinal tetras, don't cooperate and breed easily in captivity and so are caught in the wild. It has been demonstrated that well-managed fisheries for some aquarium fish (like the quick-to-mature, and naturally prolific cardinal tetra) can actually provide a good living for folks living in rural tropical areas – in fact it can pay better than farming and fishing for food, is way safer than mining for gold, and better is for the environment than cutting down the trees. The tough part is determining if a specific fish came from a well-managed fishery – which I don't have an answer for.
So why are my fish tipping? It turns out to be a phenomenon called phototaxis: cardinal tetras orient themselves based on the direction light is hitting them. In their wild jungle habitat, the sunlight is always coming from above and provides a frame of reference that the fish use to align their own, internal, up/down direction. In my tank, the afternoon sun is coming through the side so they line themselves up as if the sun was coming straight down.
I would include a picture but the algae really likes the light and has grown on everything