Characoid fishes make up 30% of the fish in the Amazon  and get most of their food from phytoplankton floating in the water. Many of them feed on fruits dropped by trees during annual floods, so perhaps you might like to experiment with a few small pieces of raspberry or strawberry. Plants that produce fruit do so because the herbivores that eat them pass the seeds in the fruit out the other end, usually a distance away from the source. This helps the plant disperse itself through habitat.
An example is the characoid Brycon guatemalensis, which eats figs dropped into the water. The advantage to the fig tree is that the seeds have a chance of being redistributed upstream . It is easy enough to float seeds downstream, but the fish provide a useful service getting them against the current. As the flood waters recede, the fish are gradually re-confined to the river channel, but the little packages they left behind over now dry land can germinate on a freshly fertilised seedbed.
Granted that you can’t reproduce annual floods in your fish room, or at least you hope not too, but it might be an interesting experiment to see what types of fruits and vegetables your characoids might take. Starve them for a day, use small chopped pieces appropriate to the size of the fish, and be sure to siphon out any uneaten food. If you do try such experiments, be sure to write them up for your club bulletin.
One thing any aquarium writer soon learns is to never say never. There are all kinds of recipes for spawning fish, but no guarantee that any of them will work, although in most cases they will certainly help. What is often forgotten is that fish are not locked into an unvarying life style. They can’t be, since their habitat varies too much, and they must be able to quickly adapt. Those that don’t will soon be eliminated from the gene pool. Many aquarists go to a lot of trouble to set up water chemistry and aquascaping to get their fish to spawn. The catch seems to be to get them in the mood and try to simulate (and stimulate) their spawning season. While it is true that most fish spawn seasonally, it is not necessarily because it is compulsory for them, only that the habitat regulates when they can go. If the habitat allows continuous spawning, most commonly in hydroelectric reservoirs, then the fish will take advantage of that. An example is the Brazilian piranha Serrasalmus spilopleura, which changes its behaviour to spawn continuously in reservoirs .
For the aquarist trying to spawn a characoid in the home tank, don’t worry about the book saying that a species will only spawn once a year under specialized conditions. Try to think things through for your fish. Is it big and healthy? Eating well? Not in a too-small tank? Many aquarists are actually already spawning their fish but never see the results because the eggs are quickly eaten within minutes or a few hours because the tank was too small and the aquascaping lacked hiding places for the eggs and fry.
AVOIDING BEING EATEN
Alarm pheromones are chemicals given off by many species of fish if their skin is broken, such as when the fish gets chomped through (or nearly so) by a predator. Other fish detect the scent of the chemical and react accordingly. This may be redundant if the predator was seen by the rest of the shoal to which the unfortunate victim belonged, but it never hurts to be certain. In any event, pheromones are useful in conditions of poor visibility such as muddy water or nightfall.
Prey fish do not necessarily panic and scatter if alarm pheromone has been detected, but they will certainly go on red alert. One interesting study on glowlight tetras (Hemigrammus erythrozonus) revealed that if these fish detect alarm pheromones from their conspecifics, they will begin flicking their fins . This activity has two purposes. Firstly, it serves as an additional alarm signal to the rest of the shoal, particularly the ones out of range or upstream of the pheromone flow. Secondly, it also notifies the predator that the shoal is alert, thereby reducing the chances of an attack, since predators generally rely on surprise. The study showed that cichlids slowed their attack rate on fin-flickers. It is not always effective, but every little bit helps.
1] Brown, G.E., J.J. Godin, and J. Pedersen (1999) Fin-flicking behaviour: a visual antipredator alarm signal in a characin fish, Hemigrammus erythrozonus. ANIMAL BEHAVIOUR 58:469-475
2] Lamas, I.R., and A.L. Godinho (1996) Reproduction in the piranha Serrasalmus spilopleura, a neotropical fish with an unusual pattern of sexual maturity. ENVIRONMENTAL BIOLOGY OF FISHES 45:161-168
3] Araujo-Lima, C.A.R.M., B.R. Forsberg, R. Victoria, and L. Martinelli (1986) Energy sources for detritivorous fishes in the Amazon. SCIENCE 234:1256-1258
4] Horn, M.H. (1997) Evidence for dispersal of fig seeds by the fruit-eating characid fish Brycon guatemalensis in a Costa Rican tropical rain forest. OECOLOGIA 109:259-264 ?