Why is my tetra attacking other tetras when they swim nearby?
I have 8 white skirt tetras and 17 other community fish in my 60-gal tank.
Today, I noticed my white skirt tetra swimming against the filter flow (which I understand to be swimming "upstream") and observed that whenever another tetra got in its view or close to it, it quickly attacked it and went back swimming to its stream. What is the cause of this?
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This question is a bit like asking "Why does Sally like Pizza" and thus I'm a bit reluctant to answer it as all animals have their own personalities.
Nevertheless, it's likely that your fish is stressed, or just a psychopath.
You say your school size is 8, while usually a minimum of 6 is recommended for schooling fish, after living in a large school at the pet store, it is likely the fish is feeling a bit naked and unprotected, and thus is feeling at bay.
Moreover, fish like to swim upstream to get more water-flow to flow over their gills, and thus more oxygen into their blood. Once again this is a sign of stress, and could also mean the oxygen levels in your tank are too low.
Many schooling tetras squabble quite a bit among themselves: the attack sounds more like a fairly typical 'charge and intimidate' move rather than real aggression, and it might not even be connected to the swimming behavior. Unless you're seeing actual injury result from it (most likely along the fins, scales, or eyes), it's not a big problem. You'll probably see this kind of roughhousing among the other tetras at some point too.
As for the swimming behavior, fish just do this sometimes. Fish that normally spend a lot of time moving often swim into currents to force more water across their gills. It's not a behavior that's a big warning sign by itself, like flashing against the gravel or swimming upside-down. White skirts (aka black skirts) are fairly active tetras, so it might just think it's on its way somewhere new and exciting. For the most part, this will usually falls into the class of harmless fish activities that make perfect sense to them, and none whatsoever to humans.
If the fish isn't just trying to explore, there are a lot of things this could be, some scary and some not. It might be having trouble breathing, either because of water quality problems or some individual disease/deformity. It may be irritated by some external parasite. It might have lost a big fight with a higher-status shoalmate. Impossible to say for sure.
One thing I'm not clear on is whether this is new or recurring behavior, and whether it's continuous or sporadic.
Changes in established behavior are always important to watch, because they're often symptoms of some other change you don't know about yet. But if I remember your other questions correctly, I think this tank was set up about a month ago, right? So it's a bit early for them to have completely settled in yet.
If you've just seen this behavior once, it's likely that it's just a fish being weird. If this fish has stopped doing anything else though, it may be a symptom of something. Keep an eye an all your fish for any other changes over the next few days: changes in body (darkening) or gill (lightening) color, white spots or patches (which will obviously be hard to spot on this particular species), bloody streaks, loss of appetite, weak or spiraling swimming patterns, and so on. As long as you don't see any of those, there's not much cause for concern.
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