Dry and rainy season in the tank

Simulate the seasons to trigger your fish to breed

Table of Contents

    by Kristian AdolfssonMAF (Malmö Akvarieförening).

    This article was originally published in 2001! I'm working on a re-write to update it.


    Many fish from tropical areas have seasons when they spawn due to changes in their natural environment. Most often they spawn when the rainy season begins, because it brings increased food supply and increased possibility for the fry to find food and shelter.

    To try to re-create as many of the changes as possible that occur during the rainy season's beginning might be one way to spawn species that otherwise are very difficult to spawn. Many species are so easy to spawn you need not use these, in many cases cumbersome methods that are described below, but certain species and groups of species might need it.

    Begin with the general rules for breeding a certain species or group of species. If you don't succeed, try with the suggestions below.

    The following is a compilation of a way to breed fish that come from areas with marked dry and rainy seasons i.e. the Amazon and Rio Negro areas in South America.

    Data and ideas have been collected from a lot of different sources, both from books, friends and the Internet, and are based on my own experience of breeding catfishes and tetras from South America.

    This simulated dry and rainy season cycle takes about 4 weeks to go through. Using a simpler method people have been able to breed Panaque nigrolineatus, Sturisoma sp., and Siamese algae eater, which are thought to be very difficult to spawn.

    Spawning triggers in nature

    Below follows a list of the different changes that can occur during the rainy season's beginning and that might trigger a species to spawn. They are not listed in any particular order and which of them a certain species needs to spawn is not fully known.

    1. Low pressure
      After a long period of high pressure in the end of the dry season the barometric pressure falls in connection with the first rain.
    2. Increased food supply
      After a starvation period during the end of the dry season the food supply increases drastically. Certain species look like skeletons during the end of the dry season and have perhaps been without food for more than a month. Certain species even eat detritus to get some nutrition.
    3. Changed food types
      During the dry season the scarce food might consist of bottom dwelling animals (red mosquito larvae) and decaying plant parts. When the rainy season starts the food changes to insects that fall down on the surface, mosquito larvae (especially white and perhaps black) and other water insects, pollen from flowers, seeds, fruits, fresh leaves and eggs and fry from other species that have begun to spawn earlier.
    4. Increased water flow
      The rain results in increased flow of water. The fish have to get more active. Some species migrate up-stream to get to calmer and more suitable spawning areas.
    5. Increased oxygen levels
      The rain that falls on the water surface increases the oxygen level in the water. The increased water flow also makes the oxygen level increase. In many cases a high oxygen level is a condition for the eggs and fry to make it during their first days.
    6. Dilution of dissolved substances in the water
      The longer the dry season lasts, the more salts, humic substances and organic material will be concentrated in the amount of water that remains. When the rain starts the concentration of the above mentioned substances decrease in the water due to dilution. The river, the stream etc. is diluted with rainwater that has zero in hardness, which lowers the hardness and often even the pH.
    7. Change of the water temperature
      The water temperature is most often lowered due to cloudiness and the cold rainwater. How big the difference is, depends on from where the fish comes. In high terrain the temperature differences are most often larger than down in the lowlands (10ºC compared to a few degrees).
    8. Change in water depth
      The increased water volume causes the water surface to rise. The water pressure at the bottom increases and the fish gets a larger swimming space vertically. The distance to the water surface will be longer for the species that goes to the surface to catch air.
    9. Spawning sites become available
      At the end of the rainy season there is often only water in the middle of the river or stream and there are very few plants or hiding places. With increasing water depth, the fish can find new flooded areas with plants, roots, tree trunks and shadows to be able to hide eggs and give the fry a better chance to hide.
    10. Changes in the light
      The amount of light and duration of light decreases due to cloudiness in connection with the rain. Certain parts of the day can be very dark during the most intense rain. With more clouds in the sky it takes longer time in the morning before it gets light and it darkens faster in the evening.
      Even the angle that the light penetrates the surface varies from one part of the year to the other. The longer from the equator, the more the variation.
      Note that certain species want almost complete darkness to spawn (they live under dense vegetation, among tree roots and in black water).
    11. Increased plant plankton level
      When the rainy season occurs the amount increases in certain waters. This is also a signal to the adult fish to spawn because there is food for smaller fry.
    12. Right time of the year
      Certain species have a very strong "biological clock" that is linked to when the rain and dry seasons occurs in their natural distribution area.
    13. Other fish spawn
      Hormones in the water from other fish that spawns might affect another species to spawn.
    14. Sound
      Even the rain's splashing against the surface might be a signal to spawn. Maybe also the sound of thunder.

    How do we simulate it in the tank?

    Below are suggestions on how to simulate the different stimuli that are listed above. Which to choose depend on which species is to be bred. Certain species might require only a few, i.e. good feeding and a water change with lower water temperature, while others need most of the items from the list. The list below follows the same order as above.

    1. Low pressure
      Many have written about their fish having spawned during low pressures. The same species might in many cases spawn during a high pressure if the right circumstances are present though. Low pressure is of course impossible to simulate in a tank, so keep an eye on the weather forecasts and start a simulated rainy season during a passage of a low pressure. A barometer might be good to have handy to check the trend for the air pressure.
    2. Increased food supply
      If the fish are in good condition when they are set to spawn they can manage to starve for several weeks. When the feeding later begins again this will trigger the instinct to spawn.
    3. Changed food types
      A change of composition of food might trigger a spawn. In some waters in South America the amount of mosquito larvae increases (especially white mosquito larvae) during the beginning of the rainy season. If you don't feed mosquito larvae during the time before setting a species to spawn and then begin to feed with them during the simulated beginning of the rainy season is a way to simulate the change.
    4. Increased water flow
      Easily solved with different forms of pumps and filters. Certain species lay their eggs close to the largest water flow in the tank, in front of the outflow of the filter.
    5. Increased oxygen levels
      Use air driven filter and air stones. One can also let a motor filter "splash" in the surface to increase the amount of oxygen. An air diffusor can also be used.
    6. Dilution of dissolved substances in the water
      Build a higher level of humic substances (i.e. peat and alder cones) and salts (fertilizer, CaCO3, MgSO4) during the simulated dry season. Later dilute with as soft water as possible when the rainy season begins (preferably RO water).
    7. Change of the water temperature
      Use submersible heaters to keep the temperature up during the dry season. Note that certain species can't take too high or too low temperatures and that certain species prefer high temperatures to spawn. These species perhaps seek out flooded grassy areas to spawn where the sun heats up the shallow waters.
      To lower the temperature one only decreases the setting on the submersible heater until it can be turned off. To further lower the temperature one might ventilate the room or put an ?ice block? in the tank.
    8. Change in water depth
      Lower the water level to 25% of normal during the dry season. Increase it to normal level during a couple of days when the rainy season begins.
    9. Spawning sites become available
      Change the plants and decorations. If no gravel is used, plant plants in pots and move caves and roots to make it a new environment more suitable for spawning.
    10. Changes in the light
      Light intensity: With several bulbs on the tank, it's easy to turn off all but one (or perhaps only use daylight). Other ways might be to put paper between the hood and the cover glass.
      Light duration: At the equator the duration of light is about 12-14 hours year round. The longer from the equator you get the larger the difference between the seasons. Shorten with 1-2 hours each in the morning and evening. Use a timer!
      Light angle: Hard to simulate in the tank.
    11. Increased plant plankton level
      Not possible to simulate easily in the tank but one might try with infusoria. Even if it does not stimulate the spawning it might be a good first food for certain species with very small fry.
    12. Right time of the year
      Wild caught fish might require that it should be during the rainy season in the area from where they come, for them to spawn in our tanks. Check exactly from where the species comes from and when the rainy season occurs there.
      Captive bred fish have most often had their sense of when it is the rainy season and when it is not reduced and might often be bred year round. The same could be true for young fish that are wild caught. If they have not experienced a rainy season it might be easier to breed them during another point of time than when they normally spawn in the nature.
    13. Other fish spawning
      Let an easily bred species spawn in the same tank as the more difficult one. It works as a natural hormone treatment. An alternative might be to let an easily spawned species spawn in a separate tank, and take water for the tank with the more difficult species from the tank where the easily species spawns.
    14. Sound
      Add water through a plexi glass plate with lots of very small holes. The drops that fall through simulate the rain beating on the water surface.

    Further ideas that are used by breeders are:

    • Filter over limestone during the simulated rainy season. Does make the water harder but it might be that the change in water chemistry that makes certain species spawn.
    • Move the well-fed fish from one tank without optimum conditions (no spawning substrate, "wrong" water parameters, many fish that are "disturbing", etc.) to a tank with the right conditions for spawning. The move itself together with all the changes that occurs might get the fish to spawn (good way to breed many tetras).

    Suggestion of a breeding scheme

    Preparations and tips

    Choose a tank with the right size for the species in question. The tank should have a volume that will be enough when only 25% of the aquarium is filled. The most important issue is that the oxygen level is kept high enough without filter and air stones. Arrange for hiding places and a few plants. The tank should look like the end of the dry season.

    Bottom substrate

    Whether to use bottom material or not can be debated. The most common is to have some kind of gravel but peat or filter floss can be used. When a bottom material is used it will help increase the surface for good bacteria to multiply.

    Advantages with bottom material:

    • some species prefer a dark bottom, other a pale one. Some pale Corydoras prefer a pale bottom.
    • many species "like" to probe around in the bottom for food
    • less risk of fungus attack on bottom dwelling fry (i.e. Corydoras)
    • eggs that fall to the bottom are harder for the parents to find and eat
    • no reflections from the bottom

    Disadvantages with bottom material:

    • Difficult to see if all food has been eaten
    • Difficult to clean without vacuuming out sand/peat

    If you don't know how the fish spawn you have to set up the tank with a little of everything. The plants can be varied with large leafed plants (Java fern, Echinodorus, Anubias, and Hydrocotyle), fine leafed (Myriophyllum, Cabomba, and Egeria), narrow leafed (Vallisneria) and other (java moss, Najas).

    Large plants can be planted in pots for easy removal. Use roots, plastic pipes of different diameters, etc. Plastic plants can be used instead of live ones. The can be easily disinfected and be cleaned from snails etc.

    • The tank should be filled with the water from the tank where the fish were before and have the same temperature. Make sure the water has been changed newly (low nitrite and nitrate levels).
    • A filter with adjustable flow should be used.
    • The light hood should be able to give a high light level.
    • The heater should be mounted along the bottom but yet be easy to adjust. Make sure it's a good quality heater that can be fully submerged.
    • Cover the sides and top with paper to avoid scaring the fish when you are moving about in the room.
    • Do not feed white or black mosquito larvae before the spawning attempt.
    • Make sure you have peat (black peat is preferable), alder cones, leaves, peat extract or whatever you want to use. Make sure that the carbonate hardness is at about 2-3 kH to avoid to low pH levels when you add the peat etc.
    • Choose healthy and mature animals in the right proportion between males and females depending on the species and put them in the breeding tank. They should be well fed to be able to survive a two-week dry season period.

    Simulation scheme

    End of rainy season

    Still some food and the water level has not started to lower.

    Day 1. Feed about 1/10 of normal. The lights should now have a level between full power and "cloudy", about 14 hours. Filter running at full speed.

    Day 2. Lower the water level about 10%, feed 1/10 of normal. Add some calcium carbonate and magnesium sulphate to raise total and carbonate hardness 1 degree each. (An alternative is to take out 20% of the water and add half the amount with hard tap water if that's available.) Add a dose of plant fertiliser according to instructions of your product (gives more dissolved salts in the water).

    Day 3. Lower the water level about 10%, skip feeding. Increase the temperature about one degree.

    Day 4. Lower the water level about 10%. Increase total and carbonate hardness 1 degree each. Feed 1/10 of normal. Put peat, alder comes, leaves, etc. in the water. Tannins etc. will be leached from these items over the coming days.

    The beginning of the dry season

    Food supply decreases and ceases. The water level and current decreases. The temperature increases in the remaining water.

    Day 5. Lower the water level about 10%, skip feeding. Increase the temperature about one degree. Decrease the flow by adjusting the filter. Check pH.

    Day 6. Lower the water level about 10%, feed 1/10 of normal.

    Day 7. Lower the water level about 10%. Increase total and carbonate hardness 1 degree each. Stop feeding until day 21. Increase the temperature about one degree.

    Day 8. Lower the water level about 10%.

    Day 9. Lower the water level about 10%. Increase total and carbonate hardness with 1 degree each. Shut off air stones. Take out the filter and clean it. Let the filter run in another tank so it has a working bacterial culture when it's needed in a week.

    Day 10. Lower the water level about 10%. The water level should be down to 25% of the tank capacity. The temperature should be around 28 degrees. Put peat, alder comes, leaves, etc. in the water. Add plant fertiliser. Increase the lighting to max. Take away any floating plants. Start an infusoria culture. Check pH.

    Day 11-19. Leave the fish in peace.

    Beginning of rainy season

    The first clouds can be seen in the sky but no rain has started to fall.

    Day 20. Clean the filter that has been working in another tank. Decrease the lighting, both the intensity and the length (down to about 10 hours). Take out the peat, leaves etc. Check the pH.

    First rain fall

    Day 21. Put the floating plants back in. Add more plants of the type the fish like for spawning. Add clean, as soft as possible, water (preferable RO), about 20% of the tank volume. The water should have about 3 degrees lower temperature than the tank. Put in the filter and run it at half speed if possible. One could try to turn off the light a couple of hours in the middle of the day to simulate thick clouds. Lower the temp on the heater 2 degrees. Feed a little twice with mosquito larvae and newly hatched brine shrimps. Add infusoria so that the water gets a slight cloudiness.

    Day 22. Add the equivalent of 20% of the tank volume. The water should be about 5 degrees lower in temperature than the tank. Run the filter at full speed and make it "splash" in the surface. Lower the temp on the heater 2 degrees. Feed a lot and often. Add infusoria so that the water gets a slight cloudiness. Add a vitamin product and plant fertiliser according to instructions of your product.

    Day 23. Add the equivalent of 20% of the tank volume. The water should be about 5 degrees lower in temperature than the tank. Add aeration at a low level. Lower the temp on the heater 2 degrees. Feed a lot. Add infusoria so that the water gets a slight cloudiness.

    Day 24. Turn off the heater if the fish can take such low temperatures. Aeration at half speed. Fill the tank. The water should be about 5 degrees lower in temperature than the tank. If you can, open a window during the night to lower the temp further. Feed a lot. Add infusoria so that the water gets a slight cloudiness.

    Height of the rainy season

    Day 25. Aeration at full speed. Change 50% of the water volume. Feed a lot.

    Day 26-?. Cary on as day 25 until they spawn!

    Breeding protocol

    Download a PDF-file (50 kB) with a breeding protocoll based on the scheme above.

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