Aquarists spend a lot of time trying to get their pet fish to spawn. When frustration sets in, reference is often made to accounts of how the species spawns in habitat. This is certainly useful information, but as many an aquarist knows, fish do not read the books about how they are supposed to do it and will often do it another way. Aquarists need to understand that most species are not as rigid in their spawning requirements as often thought. In habitat the fish may spawn according to such-and-such circumstances, but nature is often capricious and a spawning pair may suddenly have to alter their behaviour to accommodate drought, predators, or other unexpected factors.
For cyprinids, spawning is usually seasonal. Like many freshwater fish, cyprinids want to get their fry hatched at the peak season for food and living space. This peak season is normally tied to rains or food availability, generally associated with high water levels from seasonal floods. Humans, however, do not appreciate floods, and everywhere in the world are building dams that reduce variation in river and lake levels. Fish species adapted to spawning in seasonal floods must therefore adapt themselves to a lack of seasonality if they are to survive. One recent study  on the tinfoil barb (Puntius schwanenfeldii) shows this is exactly the case, as the barbs have had no choice in their native Malaysia but to switch to aseasonal reproduction. Their native rivers are dammed and floods are no longer a trigger for spawning.
This is not such a bad thing; it allows them to spawn at any time rather than once a year during the flood season. It is of practical importance to aquarists because the aquarium is not a flood-prone environment (we hope). Aquaria usually maintain the same water conditions continually, removing any cues for its inhabitants to spawn. For some species, this does matter. But others are not able to adapt to an unvarying environment because they are hardwired for specific cues to trigger spawning. The aquarist must therefore fiddle about with the tank to induce those cues if spawning is wanted. But for many fish, such as tinfoil barbs, spawning can be released from such compulsory observation of cues, and it becomes much easier for the aquarist to get fry.
The most obvious thing about cyprinids is that they swim back and forth in schools. Mostly they swim with members of their own species, although other fish of similar size are known to occasionally join up. One interesting study on tiger barbs (Puntius tetrazona) showed that they will school with images from both analogue video and computer video systems . This sounds like a fun project for the home aquarist, since both types of videos are common enough in the home, although one might have a bit of trouble lugging a tank up to the monitor or vice versa. You might want to try it for fun. Tape your fish, put the tank up against the screen, and see if a single barb will school with the video ones. If you have digitized video via your home computer, you could then fool around with colour and size changes to the video fish. See how far you can modify the video fish and still get a live one to school with it!
Schooling is an anti-predator system, and reduces the chances that a given individual will be successfully attacked by a predator. Firstly, the more fish there are, the more eyes there are to keep watch and sound the alarm, which is why so many animals, not just fish, have developed the herd mentality.
When a predator attacks a school of similar fish, the school explodes outward in a blur of rapidly moving fish that makes it difficult to track one individual. But just as lions usually take out the old gnu or the sick gazelle, so it is that parasitized cyprinids are more likely to be eliminated from the gene pool. A study on the European minnow (Phoxinus phoxinus) showed that minnows infected by larvae of the parasite Ligula intestinalis had trouble maintaining position in a school . The infested fish tended to stray out past the school, where they were easily picked off. Most fish that eat such infested minnows digest the parasitic worms as well, but some don’t, which enables the parasites to use the new host to spread further.
1] McAdam, D.S.O., N.R. Liley, and E.S.P. Tan (1999) Comparison of reproductive indicators and analysis of the reproductive seasonality of the tinfoil barb, Puntius schwanenfeldii, in the Perak River, Malaysia. ENVIRONMENTAL BIOLOGY OF FISHES 55:369-380
2] Barber, I., and F.A. Huntingford (1996) Parasite infection alters schooling behaviour: deviant positioning of helminth-infected minnows in conspecific groups. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON 263B:1095-1102
3] Clark, D.L., and K.R. Stephenson (1999) Response to video and computer-animated images by the tiger barb, Puntius tetrazona. ENVIRONMENTAL BIOLOGY OF FISHES 56:317-324 ?