Nippy Cardinal Tetras
Is it common for Cardinal Tetras to harass and nip each other? I was at an aquarium store, intending to get them after adjusting my water to their needs, when I saw this behavior. There were about 15 Cardinals in the aquarium and all of them were darting at each other and nipping at each other's bodies pretty constantly. They were definitely making contact too, they weren't just lunging and veering off. All of them were doing this. I must have watched them for at least 10 minutes, and nipping was the primary activity, they were all staying in half of the tank by each other nipping and lunging away. I was under the impression that these were very peaceful fish, but now I'm wondering about them. They were really washed out (they barely showed their red colors at all). Could they have been doing it due to some environmental factor, or is nipping at each like this other normal behavior for them?
I have a couple other types of Tetras - Black Neons and Gold Tetras - and I know that there will be a hierarchy within the school. But while I've seen my Gold Tetras chase each other they've never actually nipped each other like this. And while I've seen my Black Neons display to each other, I've never seen them actually make contact either. So I think what the Cardinals were doing, actually nipping each other's bodies, was on another level than what I've seen from these peaceful Tetra types.
I would want to actually see this before I could give you a definite answer, as the behaviour might or might not be normal. But if the fish are severely stressed, then yes, this will cause aggression. Stress has many effects on fish, worsening as it increases or continues. My article on stress goes into all this:
However, characins do establish hierarchies within their groups. And in fact, seeing increased interaction between the fish can be a good sign that the fish are suited to their environment. However, they will not be "washed out" but more intensely coloured, so this may be a clue that what you are observing in the store tank is not positive.
A couple weeks ago I acquired a group of wild-caught cardinals. After a couple days in their new home, their colours really intensified, and several of the males began what I refer to as "sparring." Pencilfish in the genus Nannostomus, which are also characins, do this a lot, and I see it from time to time among hatchetfish and tetra species. Many consider this "play," like young cats and dogs "playing," and while this is probably true I believe there is a much more serious aspect to this among the fish.
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