Do you need to vent ?
by a Petstore Staff Member

How to sex oscars and other sexually monomorphic fish, particularly Cichlids.

Determining whether or not an individual of a sexually monomorphic (male and female having no externally apparent differences in form or color)species is either male or female can be achieved by a process commonly called "venting”.This involves removing the fish from the water with a net, turning them upside down, and examining their ventral  side, looking for the genital pore-an opening located in the area where the fish releases it’s urine/feces , and also where the sperm or egg laying tube is located.Both openings are located between the ventral and anal fins.The more anterior (forward) pore is the anus, while the second pore is the urogenital opening, where the genital papilla, or vent, is located. The genital papilla (also called an ovipositor for females) is the pore through which the fish will excrete either eggs or milt (sperm).It takes a few tries to be able to accurately pick up on the difference, but once one sees a few example of males and females, it is an absolutely reliable and foolproof method.

One will see two holes between the ventral and anal fins, in a direct line, one behind the other-the size difference between the first hole and the hole closer to the anal fin, is generally smaller in a male fish, and the genital pore of a female of the same size/age will be larger than the male.Using a bright desk type goose neck lamp will facilitate viewing the difference, and patting the area dry gently with a cloth towel to remove the water/mucous clinging to the body also makes it easier to see the actual pore opening.If the vents are not clearly visible, one can also take some methylene blue or malachite green on the end of a q-tip, to  stain the openings so there's more contrast between the opening and the surrounding body tissue.

Male angelfish (Pterophyllum scalare, and Pterophyllum altum) will almost always have the tip of their sperm tube sticking out a teeny bit from the bottom of the body,once they reach sexual maturity, and if veiwed against a dark background, the tip of the tube should be easily visible on healthy individuals, as males do not need to wait for two weeks to develop eggs like the females.They are always ready to spawn. Females, on the other hand, will only 'show' their ovipositor about 3 days before the actual spawning event, and this will be preceeded by a noticeable swelling in the abdomen, from the resulting eggs developing inside the body.The larger the eggs of the particular species, the easier it is to see the size difference between the anal and genital pores.This is why, for the most part, substrate spawners are more difficult to sex via the vent method than mouthbrooding species are.(maouthbrooders as a whole have larger eggs than substrate spawners).

Generally speaking, females will have a larger genital papilla than males, due to the eggs being larger, and needing a larger size tube to pass through. In This difference is more difficult to distinguish for substrate egg layers (e.g., Neolamprologus spp.) because their eggs are smaller than those of female Mouthbrooders.

Here's a good link to fish anatomy showing the relative position of the anal and urogenital openings, and the location of the various organs in a freshwater fish's body cavity:

http://www.hedley.ca/anatomy_internal.html