I have been asked many times this summer to help sex angelfish. So, thought I'd try to explain what has taken me 20 years to learn! For starters, the younger the angelfish the more difficult it is to sex them! If it isn't bigger than a quarter in body size, don't even bother trying to sex them. You will have a 50/50 chance at getting it right; with luck of the draw being the primary factor in accuracy. Sexually mature angels are much easier to sex, but these characteristics are not cut and dry. And there will always be an angel to fools you! In fact I have one that after 3 years I still struggle to sex it!!! Even with all these traits that lean toward one sex or the other, the only way to be 100% sure is to see it breed. When the breeding tubes are out it is usually a piece of cake. Female tubes are very thick and take on more of a squared off shape. Males are very small and come to a point more like a triangular shape. What characteristics to look for Here is a list of the characteristics I look for to help determine the angel's sex: General body shape - Female: round; Male: angular Head shape - Female: flat, ski slope shaped; Male: forms a hump Eye/Nose band - Female: flat, inline with forehead shape; Male: develops a ridge at maturity Dorsals - Female: held slightly backward, in line with the forehead angle; Male: fully erect, forms almost a 90 degree angel with the head bump Ventrals - Female: tends to hold ventrals closer to the body; Male: held erect, forming a distinct angle with the body Lateral line - Female: nose tends to be slightly elevated compared to the tail; Male: nose and tail are fairly horizontal Many of these traits are fairly minor characteristics. But add them all up together and one sex or the other tends to stand out. This picture is that of my Red Capped Koi angels. These two were mated until the male died. Compare the characteristics and you will see how difficult it can be sexing angels! This is why it took me over 20 years to have a higher than average success rate with sexing them.