How to Identify Pregnant Fish

A quick online search will reveal whether the fish you're raising lays eggs or gives birth to live young. Look for signs like a swollen abdomen indicating pregnancy, or tiny, round eggs resembling jelly in the aquarium. If you plan to raise fry, it’s important to learn as much as you can about the species you're keeping, as breeding fry can be challenging.

Use this method for live-bearing fish species. Guppies, mollies, swordtails, and platies are among the most common live-bearing aquarium fish. In these species, males and females mate, and the female carries the eggs inside her body. After one or two months (for most aquarium fish), the eggs hatch into fry, and the mother gives birth to them.

• Search online for the name of the species you're keeping to determine if they are egg-layers (oviparous) or live-bearers (viviparous).

Distinguishing Male and Female Fish. As a general rule, among live-bearing fish species, males often have brighter or more vibrant colors, with longer, narrower anal fins near the tail. Females typically have duller colors, and their anal fins are more fan-shaped or triangular. If you can identify the gender of the fish, it becomes easier to tell if two fish are fighting (usually two males or two females) or mating (one male and one female).

• For species where gender is difficult to differentiate, you might need assistance from an expert at your local aquarium store.

Observe Mating Rituals. Different fish species can exhibit distinct behaviors during pairing, mating, and other related activities. For many species, including most livebearers, males often chase females enthusiastically, sometimes causing scratches, bites, or other injuries. In species like discus fish, male and female pairs work together to protect a specific area in the tank from other fish. In both cases, when actual mating occurs, the male and female may intertwine, flip over, or display other subtle behaviors that are difficult to notice.

Watch for a Swollen Belly During Pregnancy. Typically, a pregnant female fish's belly will expand, becoming round or 'boxy' within 20-40 days.

• In some species, like balloon mollies, there is a natural swelling, but it occurs towards the front, just below the gills.
• Overweight males may show swelling around the chest area. If you refrain from feeding the fish for two or three days, the swelling may shrink, while the pregnant female’s belly will remain visibly enlarged.

Look for a Red or Black Spot. Pregnant female fish often develop a 'pregnancy spot' on their abdomen, near the vent. This spot is typically black or bright red, becoming more pronounced during pregnancy.

• Some fish may always have this spot, but its color will be more vivid or darker when the fish is pregnant.

Learn How to Care for Fry. Raising fry can be incredibly challenging and often requires a separate tank to protect them from adult fish or harmful filtration systems. If you’re not yet ready for this task, consider contacting an aquarium store or an experienced hobbyist who can assist you or take the fry. If you choose to care for the fry, you can start with the guide on raising fry below, but it's also essential to research the specific species you are keeping.

This method applies to egg-laying fish species. Many aquarium fish species are egg-layers, including discus fish, bettas, and most varieties of goldfish. In these species, females lay hundreds of eggs. They typically lay them in nests made on the aquarium floor, on the walls of the tank, or on the water's surface. If there are male fish in the tank, they may fertilize the eggs after the female lays them or mate with the female beforehand, depending on the species. The eggs will hatch into fry.

• Search online to determine if the fish species you keep are egg-layers (oviparous) or livebearers (viviparous).
• Some species of fish allow the female to store sperm for several months before using it to fertilize eggs. Therefore, even if your aquarium only contains female fish, you may still observe reproductive behavior.

Observing fish nest-building signs. Some egg-laying fish create nests to protect their eggs. These nests may appear as small holes or piles of gravel, but they are not always immediately visible. Some goldfish species may build complex bubble nests, typically made by the male fish on the water's surface.

Inspecting the fish eggs. Female fish of this species may exhibit a swollen belly due to developing eggs inside, but this change is typically not significant and does not last long. The eggs typically resemble tiny round jelly-like beads. In most cases, the eggs are scattered in the water, but in some species, they may accumulate in the nesting area or stick to the tank's bottom or walls.

• Many egg-laying fish also engage in mating behaviors, including most goldfish species. These behaviors are often vigorous and can last for several hours, culminating in the egg-laying event.

Preparing for egg hatching. Caring for the fry can be a challenging task, but even without prior preparation, you will still have some time before the eggs hatch. You may want to consult with your local aquarium shop if you plan to raise fry yourself, as the care process may vary between species. If you are not prepared, refer to the following section on raising fry for basic advice, but do not expect this method to be optimal for every fish species.

Learn as much as you can about the fish species you're raising. The following guide offers basic knowledge and useful steps to take if your aquarium unexpectedly fills with fry. However, raising fry is a true challenge, and the more you understand the characteristics of your fish species, the better.

• For more specific information on particular fish species, follow these guides on breeding and raising discus fish, goldfish, betta fish, and guppies.
• Seek advice from aquarium shop staff or online fishkeeping forums. Advice from these sources is often more useful than general pet store recommendations.

Switch to a sponge filter regularly. If you're using a water pump or a filter that creates a strong current, replace it with a sponge filter from the aquarium shop. If you don't do this, the water flow could exhaust the fry or even suck them into the filter, causing them to die.

Isolate the fish. Many breeders set up a separate tank to transfer eggs or fry into. However, if you're not an experienced fishkeeper, it can be challenging to create a safe and stable environment in a short time. Instead, you can use a plastic mesh divider from the aquarium shop to separate the fish. Depending on the species, parent fish may either care for or eat the fry, so it’s important to seek advice online that fits the species you are keeping. If you can’t find specific advice, isolate the fish based on their behavior:

• If the parent fish lay eggs in a nest and protect them from other fish, separate the parents and eggs on one side of the mesh and other fish on the other side.
• If the mother lays fry or eggs in the water, keep the adult fish on one side of the mesh. The fry can swim through the mesh to avoid the adult fish.

Feed the fry with appropriate food. Sometimes you can purchase a specific "fry food" from the aquarium store, but often you need to choose from various fish foods. Infusoria, liquid fish food, or rotifers are typically safe for fry. However, as the fish grow, they will require additional food. These food types may vary depending on the species and size of the fish. Ask the staff at the aquarium shop for advice on the proper food for your fish species.

• If you can’t visit an aquarium store, you can feed the fry boiled egg yolk strained through cheesecloth.

Prepare to raise fish until they reach adulthood. Set up another tank in advance if you plan to keep some of the fish. If not, get in touch with your local aquarium store or fellow fish enthusiasts to arrange for selling or rehoming the fry once they reach a certain age.

• If you do not want your fish to breed, you must separate the males from the females. If it’s too late, contact the aquarium store as they may be able to take the fish off your hands.

• If your fish becomes bloated, sluggish, and its scales stand up, you need to seek advice from an expert or a pet store. The fish may be sick, not pregnant.
• Unless you create a suitable environment, most or all of the fry will not survive.
• Never release fish into natural rivers or lakes unless that is the exact location from where you originally brought them. Otherwise, you could unintentionally introduce harmful invasive species into the local ecosystem.

If you want to keep the fry:

• A small backup aquarium or partition net
• A small sponge filter with a pump and accessories
• Fry food
• A suitable place to house the fry as they grow and the tank becomes crowded