How to Identify Pregnant Fish

Apply this method to fish that lay eggs. Many aquarium fish are egg-laying species, including discus, bettas, and most varieties of fish. In these fishes, the female lays hundreds of eggs. They usually lay in nests on the bottom of the tank, on the sides of the tank or on the surface of the water. If there is a male fish in the tank, he can fertilize the eggs after the female spawns or mate with the female before, depending on the fish species. The eggs will hatch into fry.

• Look up the name of the fish you keep online to see if they are oviparous, or live birth (viviparous).
• In some fish species, the female has the ability to store semen for several months before using it to fertilize eggs, so a new aquarium filled with only female fish can still spawn.

Watch for signs of fish nesting. Some spawning fish create nesting areas to protect the eggs. These nests may look like small holes or piles of gravel, but are not always obvious. Some very talented fish are able to build elaborate nests out of a clump of foam, usually made by the male on the surface of the water.

Check the fish eggs. Some females of this species have enlarged bellies as eggs develop inside, but this is usually not a major change and does not last long. The eggs laid out often look like tiny round jellies. Usually fish eggs are scattered throughout the water, but in some fish the eggs can collect in the nesting area or stick to the bottom of the tank or the walls of the tank.

• Many species of spawning fish also engage in mating behaviour, including most fortune teller fish. They often show a very energetic expression, which lasts several hours and ends with egg-laying.

Prepare eggs to hatch. Caring for fry can be hard work, but even if not prepared in advance, you still have some time before the eggs hatch. You should consult an aquarium store if you plan to raise your own fry, as the rearing process can vary depending on the species of fish. If you’re not prepared, refer to the following section on raising juveniles for basic advice, but don’t expect this method to be optimal for all fish species.

Learn as much about the species of fish as you can. The guide below can provide you with the basics and helpful steps to respond if your aquarium suddenly appears full of fry. However, taking care of the fry is a real challenge, and the more you know about the characteristics of your fish, the better.

• For more detailed information about a particular fish, follow these guidelines for breeding and keeping discus, fortune teller, betta, and guppies.
• Ask for advice from an aquarium shop staff or an online forum of aquarists. Advice from those places is often more helpful than advice from a general pet store.

Replace the regular filter with a sponge filter. If you are using a filter that absorbs water or creates a flow, replace it with a sponge filter from your aquarium store. If you don’t, the current can exhaust the fry, they can even get sucked into the filter and die.

Fish isolation. Many breeders install another aquarium and transfer the eggs or fry there. However, if you are not an experienced aquarist it is difficult to create a safe and stable environment in the short term. Instead, you can use a plastic mesh net purchased at the aquarium store to isolate the fish. Depending on the species, the brood may either care for or eat the fry, so you should try to find advice online that is appropriate for the species of fish you are keeping. If this is not possible, isolate the fish based on the behavior of the brood:

• If the brood spawns in the nest and protects the eggs from other fish, use a net to separate the brood and eggs on one side and the other fish on the other.
• If the mother fish gives birth or lays eggs in the water, you need to keep the adult fish on one side of the net. The fry can swim through the net to avoid the adult fish.

Feed the fish with fry food. Sometimes you can buy a “fry food” at the aquarium store, but often you need to choose between a variety of fish food. Grassworms, liquid fish food or rotifers are generally safe for fry. However, the fish will need additional food as they grow. These types of food can vary depending on the fish species and fish size. Ask the staff at the aquarium shop to find out what kind of food for the fish you are keeping.

• If you can’t go to the aquarium store, you can feed the fry with well-boiled egg whites pressed through cheesecloth.