As I mentioned previously, I recently acquired a school of Serpae tetras. And they weren't quite what I expected (though the fault was mine for not doing better research). So a couple weeks on, what do I think of them?
[Update: These may not be Serpae tetras.]
To their credit, they're attractive, active fish. They use the entire water column (at least with the tank-mates they currently have) but seem to favour the front of the tank. (Again, that might be specific to these fish, but they certainly are aware of humans and know that we're a source of food).
They are strongly motivated by food. My Glowlight tetras have limited interest in a block of dried tubifex (they will eat the floaters dislodged by other fish, but they won't attack the block itself) and almost none in algae wafers. The Serpaes, on the other hand, will greedily attack dried tubifex and sinking algae wafers. While the Glowlights will spit out flakes that are too large for them, the Serpaes hold onto any food and eventually swallow down whatever they get a grasp on (often looking like they're at risk of choking on the food, though I suppose that in order to choke you need lungs).
As tetras go, they're fairly aggressive. While most of the aggression is directed at conspecifics, they Serpaes seem to have effectively banished both the Corys and the Glowlights from their preferred terrain (the front of the tank) except when food is present, when they seem to focus their aggression on conspecifics. The intraspecific aggression is interesting - one fish will go at another with its dorsal and anal fins expanded. Sometimes the fish that's attacked will swim off (in which case the aggressor seems to give chase) while other times they will spin around and stand their ground, responding with a similar display. The two combatants may clash a couple times before one retreats, though rarely very far. This sort of thing usually happens around food, especially algae wafers.
Tubifex blocks are very light, very low density - they're just freeze-dried worms compressed into a block. Once they fish start to attack them, they are quickly dismembered (especially if the fish in question are Serpaes or Kuhli loaches...Corys, which don't actually bite into things, can't break the block up as quickly). Once the Serpaes get to them, they don't last very long. Algae wafers, on the other hand, as fairly solid, although they gradually soften up as they soak in water. Since they last longer, they are soon covered with a scrum of Corys which completely cover the block and only offer armoured tails to the Serpaes. As a result, the Serpaes gather around above the wafer, waiting for the wafer to be exposed (which happens every now and then as the Corys move the wafer around and flip it over, and every so often seem to lose the wafer entirely). It is at this time, when the Serpaes are gathered together for a prolonged period, without clear access to the food, that I observe most aggression.
I hope that as the plants grow the sight-lines in the aquarium will be more broken up and life will become more peaceful.