Our curious minds always want to know whether the fish in our aquarium is a male or a female. Some of us are enthusiasts, while some of us simply want to know. This can be a frustrating task because researching the gender means examining individual species. In fact, determining the sex of your fish could range from easy to nearly impossible, either way we all want to be sure. Nobody likes being in doubt, right?

Here are some handy tips that will help you determine the sex of your fish:

1. Compare and Observe Their Size During Spawning Season

If you have two or more fishes, it becomes easier to determine. For many fishes like koi, the females are larger than males. However, during spawning season, the females tend to have larger abdomens, where they carry the eggs. You could quickly take a picture of each of them or catch any one of them in a net and use a ruler to measure its width and length. You’ll be able to get a general idea about the length and width of the fish.

2. Look for a Bump on Their Forehead

If you notice carefully for a large, protruding bump between the eyes and above their mouth, chances are that the fish is a male. This bump is known as “nuchal hump”, which is present on fishes like tilapia, Oscar, discus and angelfish. Many species might not have the nuchal hump, but if your fish does, be sure that your fish is a male.

3. Look out How they Grow During Mating Season

There’s something very interesting about male fishes and that is, they grow bristles, bumps and shiny scales during the breeding season in order to attract other female fishes. Female fishes might also grow these qualities, but the male versions are more obvious and attract more attention. For instance, a male pleco fish will grow longer bristles than female ones. The bristles usually grow around the edge of their mouth and head. This can help you identify the gender of the fishes.

4. Notice Triangular Fins Near the tail of Guppies and Mollies

If you see a small fin in triangular shape near the back of your small fish, you may successfully identify them as a male. Just so you know, the triangular fin is known as “anal fin” and is used for sexual reproduction.  For guppies and mollies, you might need a magnifying glass to see this triangular fin.

5. Male Pleco Fishes are More Aggressive

There’s no justification why pleco is popular. They look great in an aquarium. However, if you notice any aggressive behaviour of your pleco towards other fishes, know that it’s a male! Male plecos are notorious; all they do is scare away other fishes that come near them.

6. Seasonal bumps on goldfishes

If you are a fish lover, you must have a goldfish in your aquarium. During springtime, take a look at the gills, forehead and pectoral fins of your goldfish. If you happen to see small clusters of bumps in those areas, your goldfish is most likely a male. The bumps disappear during the springtime when the breeding season is over. The bumps vary in different sizes, colours and shapes, but lean towards a small area on fishes.

7. Visit a local pet store

You can simply walk into a pet store and consult someone who can direct you about fishes. They always have an expert among them, who can help you out with the gender of your fish. Avoid removing or replacing your fish from the tank that they stay in as it can cause stress in them and thus kill them. The pet store consultants can also advise you on how you can breed male or female fishes.

sex of the golden fishes

Additional Tips

  • Watch out for the breeding activities such as laying eggs. If all the other ways are exhausted with no avail, this could be the only possible way to determine your fish’s gender.
  • Your fishes need to be sexually mature. Although it varies on different species, waiting for at least a few months gives a more exact result on the gender of your fish. Most fishes don’t display obvious differences between genders.
  • Notice their colouration as it may sometimes denote the difference between male and female. Male fishes are much brighter in colour than female counterparts. Plus, males are cheekily coloured in orange or blue, while females are normally more demure
  • If you have a couple of fishes in your tank, compare their size from one to another. This is always an easier and better way to judge, but do keep an eye on the breed of fish you have. In some breeds, the female is larger than the male, or the belly region is more rounded.

Sex Determination

An interesting scientific explanation says that the sex of a fish can be predetermined by outside factors.

  • Environmental Sexual Determination: At an optimal temperature for fishes, there’s an equal chance of a male and female of being produced. The ideal temperature they are exposed to, the faster the offspring are to a gender ratio of 1:1. Temperature not only affects the gender but also impacts the incubation period, nesting intervals, growth and activity rates and species distribution.
  • Genotypic Sexual Determination: While we think that the sex of the child of a fish is determined by other factors, in reality, the fertilization of the egg mates’ copies from the chromosomes of each parent. The female mammals can only provide the genetic code X (female), the rest depends on the male, whether he provides X (female) or Y (male). The probability of each child has a 50/50 chance of being an XX (female) or XY (male). This chemical coding inherited by the child can go twisted through the incomplete transfer of information.

Manipulating Gender in Developing Fish

As ichthyologists have known that among some types of fishes, “gender” is not actually a concept. Surprisingly, in some fishes, gender can be affected by social changes during their adulthood. This is very much possible as research reveals that gene cells of fish can retain their bipotentiality to distinguish into male or female until the full sexual maturation process. Another fact claims, when a male fish expires, a female fish takes over the role of the male fish by changing her sex. They then not only behave like a male but also fertilize the eggs of other female fishes.

Research also reveals that the gender of developing eggs is directly affected by a number of factors. Depending on the type of fish, it also depends on the age of spawning adults, pH and DH of water, temperature, a mixture of chemicals, light levels, hormones and hormone-like substances in water. This clearly explains why some aquarists have problems with severely biased sex ratios in spawns from a certain type, while other aquarists are working with the same fish facing no problem at all. It could also be because of the aquarists having different water conditions.

More to be Learned

With so many variables on the sex of developing offspring may not only be a highly frustrating task for hobbyists but also cause serious problems and have significant economic effects on the commercial aquaculture industry. Moreover, further research draws conclusions that seem to indicate that freshwater, temperature and pH levels have the most direct and important effect on the sex.

Experts admit that there is so much more to be explored about the fish embryology and development. Sex determination usually happens during the egg and larval stages of development, with factors including mass population and water temperature, pH and DH having the most effects. However, the sex of your fish is not always determined by one or two factors exclusively, rather a combination of factors influence their gender as a whole.

Sometimes, you could be faced with unfortunate consequences such as a skewed gender ratio in a batch of fry. By the time you could think of a solution, it might be too late. But when your fish produce the next batch of spawns, you could try a little experimentation.

Separate the fish eggs into two or more tanks which will result in a number of pairs (one female, one male).

  • Modify the pH level of the water
  • Lower or raise the temperature
  • Change the water hardness by adding more hard or soft water
  • Try to change the water more frequently
  • Just give the fish more room

Bottom Line

While you want to find out the gender of your fish, ensure that they are safe. Try not to keep them out of water for more than 30 seconds and try not to hurt them in any way attempting to figure out about them.

So, were you able to determine the sex of your fish? Let us know in the comments below.

Olivia Williams
Olivia is our head of content for MyPetNeedsThat.com, mum of one and a true animal lover. With 12 different types of animal in her family, it's never a dull moment. When she isn't walking the dogs, feeding the cats or playing with her pet Parrot Charlie, you will find her product researching and keeping the site freshly updated with the latest products for your pets!

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