Introduction to Freshwater Fish
Breeding in aquarium
One of the most challenging yet rewarding aspects of fishkeeping is to breed your own fish. Think carefully about which fish you should breed and where you will house the spawning fish, and decide how many fry you can comfortably cope with and how you will feed them. Make a record of your successes and failures so that future breeding attempts go more smoothly.
Many fish spawn in pairs, but some species form small breeding groups—such as a male and two females, or vice versa—while others spawn communally in shoals. You will need to sex your stock in order to be sure that you have fish of each sex. Sexing is easy with species that display sexual dimorphism (see p.61). However, some fish, such as barbs and tetras, are visually alike and hence impossible to sex outside the breeding period. Starting with at least six individuals of such species should guarantee that there is at least one pair in the group.
Compatibility—the ability of a pair to interact favorably and spawn successfully—is unlikely to be a problem with barbs, tetras, and other species in which the sexes come together only briefly to mate. However, it is more of an issue in species that display a degree of shared parental care, such as angelfish (see pp.140–141) or discus (see pp.142–143), since they necessarily spend more time with their partners. A pair of discus, for example, will simply refuse to breed if they are incompatible. Changing their partners or, better still, rearing the fish in a group so that they can choose their own mates as they mature should overcome this difficulty. By watching the behavior of the fish, you should be able to see signs of compatibility, because naturally matched pairs will swim together and remain in relatively close contact. Alternatively, you can avoid these problems altogether by buying a proven spawning pair from an aquarium store.
The males of some larger species, such as a number of the Central American cichlids, will become aggressive at spawning time. If the female is harassed by her intended partner, she may lose her breeding condition, and it may be necessary to remove the male for a period of time to enable her to recover. Incompatibility is not something to be taken lightly—in extreme cases, a male may bully a female to death.
Conditioning your fish
Having chosen your breeding stock, the next task is to condition them for spawning. This involves feeding them well, especially with protein-rich foods, so that they produce plenty of healthy sperm and eggs. For those egg-laying species that spawn seasonally in nature, it may also involve altering conditions in the tank to mimic the environmental changes that occur in the wild. With Amazonian species, which breed when heavy rains raise the water level in their habitat, spawning can be triggered by making a partial water change and dropping the water temperature slightly. Feeding extra live foods will also help, since invertebrates naturally become more numerous in the floodwaters at this time. In contrast, annual killifish can be brought into breeding condition by lowering the water level and slightly increasing the temperature, since in the wild they spawn when the sun begins to dry up the pools in which they naturally live.
It is usually possible to tell from the changing appearance and behavior of the fish that spawning is imminent. The males may take on a more intense breeding coloration and show aggression toward one another. They will actively pursue the females, whose body becomes swollen with developing eggs. In some species, you may also notice that the fish perform courtship rituals or carefully clean spawning sites, such as rocks or leaves. This is the time to transfer the fish to a spawning tank (see p.65). In territorial species that spawn in pairs or small groups, always move the females first to allow them to settle in the tank before the males are introduced. Communal spawners can be introduced as a shoal.
Livebearers need little encouragement to breed in aquariums. Like egg-layers, they benefit from protein-rich conditioning foods, but many originate from relatively stable habitats and are not seasonal spawners, so they do not need environmental changes to trigger spawning. The water temperature does, however, influence the gestation period. For example, at 77°F (25°C), gestation in guppies takes 28 days, but this figure falls to just 19 days when the temperature is raised to 90°F (32°C). The disadvantage of speeding up gestation is that higher temperatures also quicken the rest of the life cycle of the fish, thus shortening their life span.
Although most livebearers produce offspring readily, even in a community tank, few of the fry survive to adulthood because they are eagerly devoured by other tank occupants— often including their own mother. To avoid this unnecessary carnage, you can transfer the pregnant female to a breeding trap, or spawning box. The breeding trap, which hooks over the side of the tank or floats freely in the water, has a birthing chamber that houses the female. Small holes or slits in the floor or walls of the chamber allow newborn fry to escape into a secondary chamber, where they can develop in safety, out of reach of the female and separate from the main tank. Once they have grown too large to be eaten, the fry can be released into the main aquarium.
Avoid buying small breeding traps, since gravid females will become distressed if they are confined in too small a space. Do not wait until immediately before the birth before transferring the female, since this is likely to cause her to abort her brood. The ideal time to move her is about a week before the brood is due.
Most breeders prefer to use a completely separate tank for rearing the fry of livebearers. The female can give birth there in a breeding trap, before being moved back to the main aquarium after she has recovered. A special V-shaped partition can be inserted into the tank as an alternative to a breeding trap. This has a narrow gap at the apex of the V through which the fry can slip. Alternatively, a net with a wide mesh can be used to separate the female from her offspring. When the female has been removed, the fry can be reared in the tank on their own.
A power filter cannot be used safely in any tank that is to house young fish, whether livebearers or egg-layers, because small fry are likely to be sucked into the filter. Filtration must be gentle, so use a simple sponge filter instead, possibly in combination with an undergravel filter. Prime the filter in advance with a culture of beneficial bacteria so that it has time to become active before any fish are introduced. The water must be well aerated, and the heater should have a special protective cover so that the fry do not burn themselves.
Feeding the fry
An essential part of establishing a successful breeding regimen is making sure that you have sufficient stocks of the correct foods to nourish the young fish. Tiny fry will initially need to be given a specially formulated liquid fry food or microscopic aquatic creatures called infusoria. You can culture infusoria yourself by placing a glass jar containing chopped lettuce and water in bright light, perhaps on a windowsill. After a few days, the water will turn pinkish as it becomes colonized by infusoria. Small amounts of this water can then be sprayed on the surface of the rearing tank. As the fry grow, they can progress to newly hatched brine shrimp. Larger fry can be given brine shrimp as a first food and subsequently small Daphnia and ground flakes.
Young fish need to be fed two, three, or even four times a day. They are not particularly mobile at this stage, so it is vital that food is evenly distributed throughout the tank and within easy reach; otherwise their growth will be checked. The sponge filter, which should be mature by this stage, will have tiny edible particles on it that the young fish can nibble. The gentle currents that such a filter creates, running off an airline, will help to waft floating food scraps toward the fry.
Dangers of overpopulation
Successful breeding can leave you with a large number of fry to care for. Regular partial water changes, perhaps as often as once a day, will be vital to make sure that the water quality does not deteriorate as a result of accumulated waste and uneaten food. As the fry increase in size, a more efficient filtration system can be incorporated into their tank. Eventually, the fry will need to be either moved to a much larger aquarium or divided between several different tanks, to give them sufficient growing space. Overcrowding the fish may stunt their growth and induce stress-related illnesses, as well as making it more likely that there may be a sudden, and potentially fatal, decline in water quality.
Some fry have special rearing needs. A rearing tank for the fry of gouramis (see pp.109–113) and related species needs to be kept covered so that the air immediately above the surface is at approximately the same temperature as the water. This is because the fish have anabantoid organs, which allow them to breathe air directly. If the air above the water is too cold when their anabantoid organs start to growth rate, while taking care not to overfeed them. function at about three weeks of age, the fry could become fatally chilled when they try to breathe at the surface.
With territorial species, separate young males before they start to become aggressive toward the rest of the brood. Male Siamese Fighting Fish (see pp.104–106), for example, must be kept apart by the time they are three months old. Young livebearers should also be separated as soon as you can sex them, to prevent littermates from interbreeding. A female livebearer can store sperm in her body throughout her life, so any unplanned matings like this will endanger your breeding program, since you will not know for sure which male sired the fry. Avoid housing together the young of related species, such as swordtails (see pp.160–161) and platies (see pp.162–164), which will readily crossbreed, or hybridize. It is difficult to predict the appearance of hybrids, and they tend to be less attractive and less fertile than the species from which they originate.
Problems and solutions
Difficulties can crop up at any stage during the breeding cycle, starting with the failure of the fish to spawn at all. It may be that you do not have a pair, or that the fish are not yet mature enough to breed. If the fish spawn but the eggs prove to be infertile, the male of the pair could either be too old or, if he is a livebearer, have a damaged gonopodium. Swap the males of different pairs around to see if this has any effect. With egglayers such as barbs, add an extra male to the spawning tank to increase the likelihood of success. If eggs are attacked by fungus, you can treat the water with a little methylene blue to control the problem, although the presence of fungus may also be an indication that the water temperature is too low.
If the young fail to thrive, or even die off, reexamine their feeding regimen. Study the abdomens of fry with a hand lens; their bodies should be transparent enough to see if there is food in the gut. If there is not, the food you are giving the fry may be too large for them to eat. The fry may also perish if the water quality deteriorates, so monitor this closely during the rearing period. There will inevitably be a few deformed fry in any brood, and these should be humanely culled.
- Thoroughly research the breeding habits of your fish.
- When choosing breeding stock, select young, healthy adults, with good markings, fin shape, and coloring.
- With fish that spawn in small groups or shoals, rather than in pairs, make sure that you have a large enough group, with the right gender mix, to ensure breeding success.
- Successful breeding may result in more fry than you can adequately care for, so find alternative homes for surplus fry in advance.
- Avoid allowing different species to breed together (hybridize). Fellow breeders will have little interest in the resulting offspring.
SPAWNING TANKS FOR EGG-LAYING SPECIES
The spawning tank must reflect the breeding habits of the fish it is to house. Include plants, among which the female can seek refuge if the male becomes aggressive. If the adults are likely to eat their eggs or fry, they should be transferred back to the main aquarium after spawning, leaving the eggs to hatch on their own. Filtration in this type of tank is gentle, so it is best not to feed the adults while they are in the tank, to avoid polluting the water unnecessarily
- A layer of marbles on the tank floor will help to prevent the adult fish from eating the eggs, which will fall between the marbles and out of reach. Alternatively, a mesh net across the tank can be used to let the eggs fall safely through.
- Fish that deposit eggs on plant leaves can be persuaded to lay them on an artificial spawning mop instead. With fish that spawn over several days, the mop can be replaced regularly and the eggs hatched safely in a separate tank.
- These fish require an artificial cave, such as a clean, partially buried clay flowerpot or a section of coconut shell. Alternatively, you can build a cave out of rocks. Cave-spawners may also breed successfully in the main aquarium.
- A soft peaty substrate is essential for fish that bury their eggs. After spawning, the peat, complete with eggs, can be removed and stored in a warm, dark place. Immersing the peat in tank water again will cause the eggs to hatch.
- A glass lid will prevent drafts from damaging the nest or chilling the eggs and keep the air over the water warm and humid. Provide tall plants to which the nest can be attached. These fish may also breed successfully in the main aquarium.
NEWLY HATCHED FRY
After hatching, a young fish is initially sustained by nutrients in the remains of its yolk sac, which attaches to the underside of the fish’s body. In this picture of Arawana fry (see p.182), the yolk sacs are the reddish-orange “bags” dangling beneath the fish. Only when the yolk sac has been fully absorbed will a fish start to swim around the aquarium actively seeking food. Until that time, it rests on the floor of the tank or elsewhere out of sight. The fry that emerge from the eggs are usually tiny replicas of the adult fish. In a few species, such as discus, the young have a body shape very different from their parents’ but come to resemble them as they increase in size.
FRY SIZE AND REARING
Mouth-brooders, such as the Pearl of Likoma cichlid (upper image), produce fewer but proportionately larger offspring than egg-laying species, such as the Firemouth Cichlid (lower image). Being larger, young mouth-brooders are easier to feed, and since there are fewer of them, there is less risk of overcrowding the tank or polluting the water. You may periodically have to remove the largest, fastest-growing members of a brood so that they do not cannibalize smaller siblings.
Most young fish, even those that are vegetarian in later life, need animal protein during the first week or so after they become freeswimming. The most popular rearing food for fry in home aquariums is the larvae of brine shrimp (Artemia species), which are also known as nauplii. It is important to set up your brine shrimp hatchery in advance so that you can be sure of having enough food for the arrival of the young fish.
- Brine shrimp eggs are sold in airtight containers. The eggs absorb atmospheric moisture readily, so avoid exposing them to the air before you need to use them. Very few will hatch if they become too moist.
- Hatch the eggs in a breeding bottle. Add saltwater (made with marine salt) and oxygenate it via an airstone and airline. The bottle can be attached to the side of the tank with suckers if necessary.
- Hatching takes about a day at 77°F (25°C). Sieve the nauplii from their empty shells. Before giving the nauplii to the fry, dip them briefly in dechlorinated freshwater to wash off salty residues.
GROWTH AND DEVELOPMENT OF A PLATY
Young livebearers, such as platies, are free-swimming at birth and have a remarkably consistent growth rate, averaging up to 1/100 in (0.3 mm) per day. Initially, both sexes develop at the same pace, but the females have a growth spurt after maturity and eventually outgrow the males.
- Three weeks after birth, traces of color are starting to show on this young fish, but it is still quite inconspicuous in the tank. It is now large enough to be safe from being eaten by the other tank occupants.
- At five weeks old, the coloration of the fish is becoming clearer. The fins are proportionately larger and more elaborate. The body is less streamlined, with a more angular back and a bulkier abdomen.
- By nine weeks of age, the patterning and richness of coloration are fully apparent. Now sexually mature, the fish can be identified as a male by its gon opodium, which is just visible behind the pelvic fin.