Environment plays a major role in fish aggression. Tank size has a lot to do with this, as well as how the tank is aquascaped, water parameters, and other fish species included. With shoaling fish, there is also the aspect of how many of the species needs to be present, and this can vary with the species.
Taking the last aspect first, shoaling fish need a group for several reasons, one of which is interaction. The species may have specific social behaviours, a pecking order, dominant/submissive roles, etc. Recent studies have proven beyond doubt that when shoaling fish are maintained in too small a group (one study used five as the absolute minimum) aggression is almost certain to occur. Normally-aggressive species heighten their aggressive tendencies, while normally peaceful species can become nippy and more like bullies not only to their own but other species as well. Think of this as the fish's means of responding to what it finds intolerable; it simply "lashes out" out of frustration. Sometimes it may respond the opposite--by withdrawing so much that it simply wastes away.
To the environment issue. The same increased aggressive behaviours were observed when fish were kept in too small a space. Bare tanks also bring out aggression. Inappropriate water parameters (temperature, pH and hardness) can also affect aggression. And having the wrong fish as tankmates--putting a fish like a
Serpae Tetra with inherent nipping tendencies in a tank with angelfish is only asking for trouble because the slow sedate angelfish with its trailing fins is too much of a temptation.
Howard Goldstein authored an article in the December
TFH entitled
Community Fish Cautions. He lists several frequently-seen and so-called "community fish" that can create havoc. But he makes a very valid point first up: "
It's Not the Fish; It's the Tank. These fish are not difficult to keep. Their required conditions are not hard to create and maintain, but they must be met."
To return to your
Silvertip Tetra. A group of 8 or more in a spacious tank at least 3 feet in length with lots of plants for cover but also space to swim lengthwise and with a slight current from the filter will likely be model citizens. Put the same fish in a 20g high and they may nip each other repeatedly. This is applicable to many species.
A last word on what one perceives in fish store tanks. This cannot always--in truth, rarely should--be taken as indicative of the fish's behaviour or compatibility. What appears on the surface to "work" in the store tank may in fact be so harmful to the fish that they are irreparably weakened. Over-crowding a species sometimes stresses the fish so much that they behave just the opposite. They are being forced into a situation that is not to their liking, and cannot respond normally. Place them in the proper environment at home, and the normal tendencies will return, sometimes. Sometimes the stress may be so great that the fish is permanently disabled, so to speak. And this works both ways; peaceful fish may become aggressive, whereas mildly-aggressive may be so docile they waste away.
Byron.
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