This is a question that many fish owners ask. The answer really depends on the type of fish in question. Not all fish will have the same gender specific traits. For this reason it may be necessary to research the exact species of fish you have. However, there are many common fish that people keep for pets so in the following article we will focus primarily on them. Keep in mind that some fish fall into the same category of species but are called by different names. It may help if you know the category your pet fish falls into when trying to determine its sex.
Betta fish, often referred to as Japanese fighting fish, are beautiful and colorful fish that are usually kept alone or only with a member of the opposite sex. This is because they will literally kill another fish of the same gender. It is not uncommon for a novice Betta fish owner to walk into a pet store, walk out with two Betta fish, put them in the same bowl, only to find one floating or half eaten within a few hours. Unfortunately, not all pet store employees will have the presence of mind to share this information. This makes determining their sex quite important from the very start. Keep in mind that typically male Bettas are what you will find in pet shops. It can be very difficult to tell the difference between and male and female Betta fish and it can take years of experience to be able to automatically tell the difference. For the most part a female Betta fish will be smaller with shorter fins. But there are other characteristics to look for as there are also short finned male Betta fish. A white spot which is actually an egg spot can be seen on the female Betta. In addition, she will not have what is considered a beard as the males do.
This is a very hard fish to sex correctly. In fact most people who are looking to buy Angelfish for breeding purposes buy a tank full of them, perhaps a dozen or even two (depending on the size of the aquarium) and then allow them to mature together. Once they become adults they will choose mates and pair off. When this happens it will be easier to determine your breeding pairs. Of course you may only wind up with one or two breeding pairs at the end but at least thats a start. Once in a blue moon you will see a slight nuchal hump on a male Angelfish and this will let you know his gender. However, this is so rare that it is not a good method of determining the sex of your Angelfish. If you are just looking to buy Angelfish because they are beautiful then the gender shouldnt matter very much. If you end up with babies then that will be a plus.
There are so many different species of Cichlids that it would not be possible or practical to try and tell you how to sex each and every one. However, with that being said there are a few things that can be used as a general gauge for determining the sex of this species. In most cases males are slimmer than the females yet quite a bit larger and much more brightly colored. In addition, male fish of this type typically appear to be more flowing. In some of the different species a male Cichlid will have a bump in his head which is known as a nuchal hump and/or egg shaped patterns on his anal fin. Please be aware that not all of the types of Cichlids will have these characteristics and this information is just meant as a rule of thumb. If breeding is your goal then it will be in your best interest to do a good bit of homework and research on the specific type of Cichlid you are interested in breeding.
As with Cichlids, Cyprinds are also a group of fish that carry a large number of different species. They are also very hard to sex. The gender difference will depend largely on the exact species within this group that you are looking to sex. One rule of thumb to follow is that male Cyprinds, like many other species of animals are usually much more vibrantly colored and slimmer than the females. One good way to tell if you have a breeding pair is to buy many of them at one time. Cyprinds are a type of schooling fish so they will eventually find mates and pair off. Obviously, if you are not interested in breeding then this will not be a major issue for you but if you are then it may take quite awhile for you to obtain your pair.
Unfortunately, if you are looking to breed Catfish you are going to have quite a time of figuring out male from female as it is all but impossible to do so. There really are no gender specific markings or rules of thumb to follow when dealing with Catfish. This is primarily because they have not really been bred in captivity. This means that experts have not really gathered enough information on this particular fish to be able to educate anyone on determining the difference. If you want to breed Catfish then you will need a pond with loads of them. There is however one species in the Catfish family that has been bred in captivity and this is the Corydoras. However, again there is really no way to tell the difference in gender until someone comes up obviously pregnant.