Developing embryos and newly hatched larvae (fry) are the most sensitive and delicate of the developmental stages in the life of a rainbowfish. Therefore, great care must be taken to provide them with the proper incubating and hatching environment. Water temperature, light, and water quality are very important.
Whether you have an unintentional spawning in your aquarium or a well-planned breeding program, to raise rainbowfish fry successfully you must be able to provide the right conditions. High mortality rates can often occur, especially during the early feeding stages. Fry mortality can be the result of several factors including inbreeding, inferior water conditions and improper incubation conditions, but poor nutrition may play a major role as well.
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Rainbowfish eggs on Utricularia gibba - photo © Günther Schmida |
Collecting Eggs
Collecting eggs from the breeding aquarium is the best way to ensue the largest number of offspring from your selected breeders. For those species that spawn a large number of eggs each day, best results will be achieved by providing them with spawning mops and then exchanging the mops and placing the spawn-laden mops into another aquarium. This could be a previously established aquarium if you want the fry to hatch out and grow up in that aquarium. Otherwise, use a bare nursery aquarium.
For those species that only spawn a small number of eggs each day the simplest method would be to just hand pick the eggs from the mops or aquatic plants. Rainbowfish eggs are reasonably hard-shelled and can easily be gathered from the spawning mops with clean fingertips. Spawn-laden mops should be softly squeezed or allowed to drip nearly dry. Individual eggs will stand out like tiny glass beads against darker-coloured strands of a mop. Place the eggs into clean plastic hatching containers for incubation. If the eggs are incubated in hatching trays, the larval fry should be transferred into the nursery tank immediately after hatching.
Egg Incubation & Hatching
A wide variety of devices and methods are used for incubating rainbowfish eggs. Depending on your needs, you can utilise a system of several small incubators attached to a common supply line or a few large rearing tanks.
The simplest method is to just place them in a small shallow tray or container filled with pre-conditioned water. A gently movement of the hatching water with an Airstone should be provided to insure adequate water circulation to all the eggs. This will prevent the accumulation of waste products, and allow gas exchange between the egg and the surrounding water. Changing a little water each day is also helpful. The hatching container can be floated in the tank where the fry will be reared. The fry will swim at the water surface of the container after hatching and can be gently poured out of the hatching container. This eliminates the need for handling the fry.
The amount and incidence of light received during incubation can affect both fish development and larval survival. Therefore, all types of egg incubating containers should be stored in such a manner to protect developing embryos from direct light.
Because of their size and permeability, fish embryos and larvae are susceptible to many types of organic or inorganic materials dissolved or suspended in the water. These may include gases, minerals, metals, and particulate matter. It is essential to know the water quality standards for embryos and larvae of the particular fish species.
Hatching temperature has a direct effect on the timing of embryonic development and is delayed at low temperatures, and accelerated at high temperatures. Hatching temperatures are also thought to modify the behaviour of larvae and determine certain morphological characteristics. There is an optimum temperature required for each developmental life stages, and these vary amongst species. Water temperatures should be maintained with minimal fluctuations.
In general, optimum temperatures for spawning, hatching, and rearing newly hatched rainbowfish species are 24-28°C. Avoid temperatures above or below this range. Poor embryo survival, low hatch success, reduced growth rates, larval deformities, and increase in fry/larvae diseases may result from temperature fluctuations or temperatures outside the optimum range for the species. If needed, the hatching container can be floated in a heated aquarium to maintain correct temperature.
Hatching time for most rainbowfish species is around 6 to 9 days at 25° Celsius. Fully developed embryos display prominent eyespots, and are usually referred to as being 'eyed-up'. At hatching, when the rainbowfish larva leaves the egg it is provided with a yolk sac, and from this, it derives sufficient nourishment to tide it over the first 12-36 hours of its free-living existence. The newly hatched larvae will remain attached to the spawning medium until its yolk sac has been used up before becoming free-swimming. The baby fish will then remain in the upper 1-cm water layer where they start looking for food. Therefore, you will have to commence feeding them as soon as possible.
A small percentage of eggs will fail to develop, usually because they were not fertilised in the first place. Dead (opaque) eggs rapidly develop a fluffy appearance due to fungal infection and should be removed regularly. Chemical treatment can be used to limit the infection, and involves the addition of a little Methylene blue to the water. Add sufficient Methylene blue to produce a concentration of 3 ppm. One treatment is usually all that is necessary, and should continue for 3 days.
Removing dead eggs with an eyedropper is probably more effective than chemical treatment at controlling fungus, but it can be very time-consuming. Very high fungal infections within the first 2 or 3 days after spawning typically indicate a high percentage of infertile eggs. This may be due to unfavourable conditions within the spawning tank or adult infertility. A sudden upsurge in losses later usually signals improper water conditions in the hatching container.
Nursery Tank
The ideal nursery tank should be bare with only a sponge filter. In this way the tank is easily kept clean and helps prevent any disease or water quality problems. During this time, rinse the sponge regularly under lukewarm running water to keep the surface area clean and free of blockage. A small trickle of air bubbles is sufficient because more air will agitate the water too much and the baby fish will have to fight the current. As they grow you can increase the airflow rate. To achieve the best growth and survival, an initial stocking density of not greater than one baby fish per litre of water is recommended, and water temperature should range between 24-28° Celsius.
Young fish can remain in the nursery tank until they are large enough to be transferred into a regular aquarium. However, this period should not exceed 90 days due to the increased growth, and the potential for the occurrence of adverse conditions of water quality. Generally, 95% survival should be expected at the end of the nursery period.
General maintenance consists of changing a little of their water every second day with a small siphon tube, removing any uneaten food and faeces, and adding pre-treated replacement water. Any mortalities or deformed fish should be removed regularly. Clean and disinfect all hatchery equipment with a chlorine solution, or other suitable disinfectant before using them for another batch of fry.
Mystery Snails (Pomacea bridgesi) can be a useful addition to the nursery tank as they help clean the tank of detritus and eat any surplus food, which has a very positive effect on water quality. As an added bonus, mystery snails will often breed in the tank in which the fry are being raised.
A problem often encountered when raising rainbowfish fry is a little freshwater cnidarian called hydra. They are usually tan or brown in colour and are not readily seen against a background of natural coloured gravel or on plants. Within the confines of a small fry-raising tank, these little monsters can be deadly, and can ingest a tankful of newly hatched rainbowfish fry in less than a week.
© Copyright Adrian R. Tappin
Updated January, 2002.
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