Using Breeding Nets for Tropical Fish: How to Use Fry Separation Systems and Key Precautions

One of the biggest challenges in tropical fish breeding is protecting fry from being eaten by adult fish. In many species, parent fish and other adults will consume fry, making it necessary to raise them in a separate environment. Breeding nets and isolation boxes are simple and effective methods, but without proper knowledge of how to use them, there is a risk of fry dying. This article covers the basics through advanced techniques for separating and rearing fry.

When Separation Rearing is Necessary

When there is a risk of eggs and fry being eaten

Livebearing fish like guppies, mollies, and platies tend to eat their own fry immediately after giving birth. Egg-laying fish such as tetras also frequently have other fish eat their eggs after spawning.

Breeding in a mixed-species tank

In tanks with multiple species and individuals, fry are perceived as "food." Even same-species adult fish may prey on fry.

When parental care is needed

Many cichlids (such as apistos and discus) protect their fry, but stress from attacks by other fish can cause parents to abandon their fry. In these cases, it is safer to isolate the fry along with the parents.

Main Methods for Separation Rearing

Breeding nets (breeding nets)

Mesh or cloth nets fixed to the tank wall. Since water can flow between the inside and outside, oxygen, temperature, and water quality remain the same as the main tank—a major advantage. Best suited for small numbers of fry or temporary isolation.

Note: Fine mesh can cause oxygen depletion (adding an air stone inside helps). Too cramped for large quantities of fry.

Isolation box (floating divider)

A small plastic box that floats inside the tank. More rigid than breeding nets and easier to manage. Perforated-bottom types allow water flow, while mesh-bottom types provide complete isolation.

Rearing in a separate tank

The most reliable method. Set up a 10–20 liter dedicated fry tank, using water and bacteria from the parent tank to establish it. Using a sponge filter prevents fry from being sucked in and dying.

Separate tank rearing is best for large quantities of fry or rare species.

Water Quality and Oxygen Management After Separation

Filtration

Water flow inside breeding nets and isolation boxes tends to be weak, causing oxygen depletion. Install a small air stone inside to supply oxygen. Sponge filters are ideal—they prevent fry from being sucked in and also provide biological filtration.

Water quality management

Fry are more sensitive to water quality changes than adults. Special attention must be paid to ammonia and nitrite, as even small volumes can rapidly deteriorate due to fry waste and uneaten food.

• Perform small water changes (10–20%) daily or every other day
• Use water from the existing tank or water that is the same temperature and has been dechlorinated
• Gently remove waste and uneaten food from the bottom with a siphon

Temperature management

Fry tend to be more susceptible to low temperature and temperature fluctuations than adults. Maintain the same temperature as the parent tank and avoid sudden changes.

Fry Care by Growth Stage

Immediately after hatching to day 3 (yolk sac stage)

Newly hatched larvae have a yolk sac in the abdomen and do not need to be fed. Many species are sensitive to bright light, so keep lighting dim.

Day 3–7 (beginning initial feeding)

Once the yolk sac is absorbed, begin feeding.

For initial feeding: - Infusoria (single-celled organisms like paramecium): use commercial culture kits or collect naturally - Brine shrimp nauplii: high nutrition and the standard first-food choice - Commercial powdered fry food: convenient but watch for water quality deterioration

Feed small amounts multiple times per day.

Week 1 to 1 month

As their mouths gradually enlarge, increase food size. Progress gradually from brine shrimp to finely crushed flake food to small pellets.

1 month and beyond

Once their mouth size approaches that of an adult, transition to regular adult food. When fry reach 60–70% of adult size, they can be moved to the main tank with low risk of being eaten.

Fry Management for Breeders

Professional breeders managing large quantities of fry build dedicated fry-rearing systems. By combining multiple rearing tanks, automated water quality management systems, and automatic feeders, they achieve both labor efficiency and improved survival rates. Keeping detailed records of growth and survival rates allows for optimization of food types, temperature, and stocking density—with each generation, management techniques improve further.