The actual triggers are the decrease in temperature, decrease in water hardness, and cleaner water. As light increases in the spring, they begin breeding. These species may need their light timers changed with the seasons to breed if kept inside. Other fish like danios and live bearers are triggered almost entirely by an increase in water temperature. Labyrinth fish often are triggered to breed by a decrease in water depth.
Most fish require a combination of changes to initiate spawning.
Each species is different so it is important to know that species' spawning triggers. Another way to initiate spawning is to feed the fish live foods like brine shrimp , tubifex or black worms, daphnia , etc. This mimics the abundance of live foods in spring and also gives the fish the extra nutrition required to produce vast amounts of eggs and sperm. Feeding fish to induce spawning is called conditioning. When re-joined, they are often ready to spawn.