In the wild, there are certain environmental signals, or triggers, that tell the fish it's time to spawn. Often, these are linked to seasonal changes that in turn set off a series of environmental changes: For instance, the arrival of the rainy season in the tropics may cause the pH, hardness, and temperature of lakes and rivers to drop and their depth to increase; they also flood their banks, creating shallows that are warmed by the sun.

Vegetation flourishes, as do insects and crustaceans, all of which provide a sudden change in diet for fish. There may also be barometric changes, higher oxygen levels in the water, and increased water flow. Some or all of these changes may stimulate physical changes in fish necessary for the production of eggs and sperm.

 Even in captivity, such environmental triggers may be required to get some species to breed. But because no obvious climate shifts take place in the home aquarium, breeders must come up with ways to simulate these environmental cues. Among the common triggers and ways to mimic them in captivity:

 Changes in water chemistry

Most often the trigger is a drop in pH or hardness. The easiest way to simulate it is to do a water change-or a number of changes over a period of time-using water with the desired values.

 Changes in depth

The trigger can be an increase, a decrease, or both. Obviously,   this is simple to simulate by adding or removing tank water.

 Changes in the light cycle

The trigger is usually increased hours of daylight and is easy to simulate by adjusting the tank lights. Morning sun also triggers spawning in some fish; place their tank where it will receive natural sunlight.

 Increased oxygenation

The colder, faster-moving waters typical of a rainy season in the tropics carry more oxygen than does warmer, slower water. An air pump and airstone can give the same result in a breeding tank.


Sudden, violent rainstorms trigger some fish to spawn. Some breeders actually simulate them for their fish, providing "rain" via watering cans, and lightning and thunder through flashing strobe lights and aluminum pans banged into microphones. Although many swear this works, to date no scientific studies bear this out. Rather, many scientists believe fish respond not to the visible (and audible) aspects of the storms, but to the changes in barometric pressure that accompany them-something a bit harder to simulate in the home aquarium! Instead, try setting your fish up to spawn when you know a storm is imminent. (However, if you are trying to breed a recalcitrant species and want to try simulating one, it certainly won't hurt!)

 Changes in temperature-Dropping the temperature by two or three degrees overnight, then raising it again in the morning, can trigger spawning in some species. Some will also respond to a water change using water with a lower temperature than that of the tank.

 Changes in diet

Mother Nature has programmed most fish to breed at times when plenty of food will be available for their fry. From an evolutionary   standpoint,   that makes a lot of sense: If there weren't enough food, the fry-and ultimately the species-would be at risk of dying out. So it's no surprise that a sudden influx of food or the appearance of certain types of food-usually live foods-can serve as a trigger, telling fish it's time to spawn. For now, just keep in mind that what you feed your fish is one of the most important factors in their ability to produce healthy fry.