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Whether you have an unintentional spawning in your aquarium or a well-planned breeding program, to raise rainbowfishes successfully you must be able to provide the right conditions. 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 a proper incubating and hatching environment. High mortality rates can often occur, especially during the early stages. Mortality can be the result of several factors including inbreeding, inferior water conditions, improper incubation conditions and poor nutrition. Water conditions that are less than optimal can cause slower growth, increased size variation, lower survival and increased incidence of disease. Hatching rates and survival can be increased using artificial incubation. Collecting eggs from the breeding aquarium is the best way to ensue the largest number of offspring from your selected breeders. Also, removal of the eggs may increase egg production by shortening the time for another spawning to occur.
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| Rainbowfish eggs on Utricularia gibba - photo© Gunther Schmida |
Collecting Eggs
Rainbowfish eggs are adhesive, negatively buoyant in freshwater and average 1.5 ± 0.5 mm in diameter; are usually clear to light amber in colour and hang by a fine thread. 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 for hatching. This could be a previously established aquarium if you want the fish to hatch out and grow up in that aquarium. Otherwise, use a bare nursery aquarium.
Spawning mops are used primarily for the adhesive eggs of rainbowfishes. By simulating a spawning substrate (plants, etc.), they serve as egg collectors and provide a place for egg attachment. Egg-filled mops are removed from the spawning tank and placed in other tanks for incubation and larval rearing. Eggs are best left attached to the spawning medium to minimise handling stress.
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 spawning mops. Rainbowfish eggs are reasonably hard-shelled and can easily be collected from the spawning mop with clean fingertips. Any eggs that burst while being collected will most likely be infertile. Spawn-laden mops should be softly squeezed or allowed to drip dry. Individual eggs will stand out like tiny glass beads against the darker-coloured strands of the spawning mop. Place the eggs into clean plastic hatching containers for incubation. Maintain a constant temperature ± 1°C and provide gently aeration. Some hobbyists add a small amount of fungicide, such as Methylene blue, while others consider it unnecessary. On this point it's a matter of whatever works for you. If the eggs are incubated in hatching containers, the larvae should be transferred into the nursery aquarium 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 containers attached to a common recirculating system or a few small individual hatching aquariums. 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. 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. Therefore, it is essential to provide good water quality for the embryos and larvae. The hatching container can be floated in the aquarium where the larval fish will be reared. The larvae will swim at the water surface of the container after hatching and can be gently poured out of the hatching container into the surrounding water in the aquarium. This eliminates the need for any physical handling of the larvae.
I have found that the eggs of most rainbowfish species do not hatch in daylight. Therefore, it's probably best that the incubation of rainbowfish embryos should occur in dim lighting to protect developing embryos from direct light. It's probably that the amount and incidence of light received during incubation can affect both fish development and larval survival.
The spawning of rainbowfishes, embryo development, survival, and growth of fish larvae all occur within a narrow range of water temperatures. Temperature is one of the major factors in determining the embryonic period for rainbowfishes. The development and hatching is delayed at low temperatures, and accelerated at high temperatures. Incubating temperatures are also known to modify the behaviour of larvae and determine certain morphological characteristics. There is an optimum temperature range required for each developmental life stage. Water temperatures should be maintained with minimal fluctuations. In general, optimum temperatures for spawning, hatching, and rearing newly hatched rainbowfish species are within the temperature range of 24~28°C. Avoid temperatures above or below this range. Poor embryo survival, low hatch success, reduced growth rates, larval deformities, and increase in 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 the correct temperature.
Hatching time for most rainbowfish species is around 6 to 9 days within the temperature range of 24~28°C. 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 few days of its free-living existence. This means that there is a certain period of time where it is not necessary for a larval fish to obtain their nutrition from external sources. Rainbowfish larvae can survive for as long as 8~10 days without external food sources providing that the yolk sac contained enough nutrients. However, if an egg does not hold adequate nutrients, larvae hatching from that egg would have a shorter period of unfed larval life than larvae hatching from an egg with a good supply of nutrients. At hatching, rainbowfish larvae measure around 3 to 4 mm, and are reasonably well developed. They are competent swimmers with well-developed pectoral fins and a continuous median fin fold, beginning dorsally and continuing around the tail. The larvae swim at the surface of the water, generally within the upper 1-cm water layer. The mouth is well developed and functional, and they usually begin feeding within hours of becoming free-swimming. Therefore, you will have to commence feeding them as soon as possible.
Incubation & Hatching Problems
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 at least 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, or egg collection damage.
A problem often encountered when raising rainbowfish larvae 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 nursery aquarium, these little monsters can be deadly, and can ingest a batch of newly hatched rainbowfish larvae in less than a week. Another pest that you may encounter is a free-living flatworm known as Planaria. They are a very small black or brown flatworm that look very similar to leeches and often appear in freshwater aquariums. They are generally around 3 to 5 mm long, but some grow as large as 10 mm. In a normal aquarium situation they usually don't cause any problems and probably even go unnoticed. However, if you get them in a breeding or hatching aquarium, they can destroy a whole spawning of eggs within hours. They can usually be seen crawling around on the front of the aquarium at night time after the lights have been turned off.
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 juvenile 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 juvenile fish per litre of water is recommended, and water temperature should range between 24~28° Celsius.
Juvenile 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. Mystery snails (Pomacea bridgesii) can be a useful addition to the nursery aquarium 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 juvenile fish are being raised. Clean and disinfect all hatchery equipment with a chlorine solution, or other suitable disinfectant before using them for another batch of fish.
© Copyright Adrian R. Tappin
Updated December, 2008.
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