BREEDING TECHNIQUES FOR TROPICAL FISH



Breeding aquarium fish is a significant milestone in becoming a skilled aquarist. Successfully encouraging fish to spawn is an indication that the aquarist can provide optimal conditions, allowing them to attempt keeping more challenging species. To breed a species successfully, an aquarist must distinguish between the sexes and replicate the natural conditions required to trigger spawning. Keeping a journal of successes and challenges is highly recommended for learning and improvement.

Sexing Fish

Determining the sex of a fish is essential in ensuring a viable breeding pair. Fish can generally be classified as sexually dimorphic or sexually isomorphic. In sexually dimorphic species, males and females exhibit distinct differences in primary (reproductive organs) and secondary traits such as size, shape, color (sexual dichromatism), and finnage. Males are often more colorful, larger, or have longer fins. Some of the most striking examples of sexual dimorphism can be found in Lake Malawi cichlids, killifish, and livebearers. In contrast, sexually isomorphic species show little to no external sexual differences. In many cases, the only way to distinguish sexes is by examining the genital papilla, which is only visible during spawning periods. Some isomorphic species show subtle differences, such as slightly larger males or rounder bellies in females. In certain species, external sexual differentiation remains unknown.

Selecting the Parent Fish

Once males and females have been identified, selecting the right breeding pair or group is critical. Several factors should be considered when choosing parent fish:
  1. Select fish with vibrant coloration and well-defined markings to produce attractive offspring.
  2. Only use healthy, mature fish for breeding. Unhealthy or deformed fish may pass on genetic defects.
  3. Ensure compatibility between the breeding pair. Some species require time to form bonds, and in cases like cichlids, pairs may need to be raised together for months or even years. In some species, an incompatible mate may be harassed or even killed.
  4. Avoid crossing different strains or color morphs, as hybrid offspring may be dull in appearance.
  5. Ensure both fish belong to the same species. Some cichlids and killifish have similar-looking females, increasing the risk of accidental hybridization.
Reproductive Strategies Egg-layers

Most aquarium fish are egg-layers that engage in external fertilization. Egg-laying species fall into five main categories: egg-scatterers, egg-depositors, egg-buriers, mouth-brooders, and nest-builders.
  • Egg-Scatterers: These species release adhesive or non-adhesive eggs, which fall onto the substrate, into plants, or float at the surface. They do not care for their eggs and often eat them. Many schooling fish, such as tetras and barbs, are egg-scatterers, often spawning in pairs or groups. They typically lay large numbers of small eggs, which hatch quickly.

  • Egg-Depositors: These species lay eggs on surfaces such as tank glass, wood, rocks, or plants. They generally produce fewer but larger eggs than egg-scatterers. Some egg-depositors, such as cichlids and certain catfish, actively guard their eggs, while others abandon them. Among caring species, spawning behavior is divided into:
    • Cavity Spawners – Lay eggs inside caves or crevices.
    • Open Spawners – Deposit eggs on open surfaces such as rocks or leaves.
    Parents typically defend and clean the eggs, with the fry remaining under their care for some time.

  • Egg-Buriers: These species inhabit temporary bodies of water that dry up seasonally. Most annual killifish are egg-buriers, laying their eggs in mud before perishing as the water recedes. The eggs remain dormant until rain returns, triggering hatching.

  • Mouth-Brooders: These fish carry eggs or larvae in their mouths for protection. They are divided into:
    • Ovophile Mouth-Brooders – The female collects fertilized eggs into her mouth, where they hatch.
    • Larvophile Mouth-Brooders – The female guards eggs on a surface until they hatch, then transfers the fry into her mouth for further protection.

  • Nest-Builders: These species construct nests to protect their eggs. Labyrinth fish, such as bettas and gouramis, create bubble nests from saliva-coated bubbles and plant debris. Some cichlids dig pits in the substrate as nesting sites. Nest-builders provide extensive parental care.
Livebearers

Livebearers give birth to free-swimming young. There are two types:
  • Ovoviviparous – The eggs develop and hatch inside the female before birth.
  • Viviparous – No eggs are formed; instead, the developing fry receive direct nourishment from the mother.
Livebearers, such as guppies, mollies, and swordtails, are prolific and easy to breed.

Conditioning the Parent Fish

Before breeding, parent fish should be conditioned with a varied, high-quality diet. Many species can be conditioned with nutritionally balanced flake foods, but some require live foods such as brine shrimp, insect larvae, and small invertebrates. Separating males and females during conditioning can sometimes increase spawning success, as reintroducing them often triggers courtship.

The Rearing Tank

A rearing tank is optional for species that care for their fry, but it is recommended for those that do not. If the same tank is used for breeding and rearing, the parents should be removed after spawning. The rearing tank should include:
  • A heater with protective casing.
  • A sponge filter or a filter with a sponge on the intake to prevent fry from being sucked in.
  • Plastic or live plants for shelter.
  • Three sides covered with black paper to reduce stress and minimize fungal infections.
The water in the rearing tank should closely match the conditions of the breeding tank.

Raising the Fry

Fry development varies between species, ranging from a few minutes (livebearers) to several weeks (certain egg-layers). Newly hatched larvae initially rely on their yolk sacs for nourishment. Once depleted, they become free-swimming and require appropriate food. First foods include:
  • Infusoria
  • Green water (algae-rich water)
  • Hard-boiled egg yolk (strained into fine particles)
As the fry grow, they can transition to:
  • Brine shrimp nauplii
  • Micro worms
  • Finely crushed flakes
  • Daphnia (water fleas)
For larger fry, additional foods such as insect larvae and chopped tubifex worms can be introduced. Daily water changes (25-50%) are essential to maintain water quality. Water parameters should remain stable, as fry are highly sensitive to fluctuations. Feeding should occur multiple times per day, and some species may require periodic sorting to prevent cannibalism by larger siblings.

With optimal conditions, frequent water changes, and proper nutrition, fry will grow quickly and develop into healthy juvenile fish.