This post is just for my friend Hasitha, who... err... let's say needs some help... I am being totally cheeky here and know he is just going to laugh at me. But hey, it will help clarify it for everyone else too ;)
I am going to show and explain the differences between male and female guppies. Mollies, platies and swordtails are all similar, so you can use the "boy bit" and "girl bit" information for them too.
Adult female guppies are full bodied (stocky) and only have colouration on their back tail. Please note the shape of the anal fin behind the gravid spot. The gravid spot is a dark spot near the vent, which is actually the babies inside the mummy. The darker it gets the more near to giving birth she is.
The vent, btw, is where the fish poop (those long, usually dark, worm looking things coming out of the back of the fish. Don't laugh - I can't count the number of times newbie fish keepers have asked me why there are "worms" in the tank.)
Here is a picture of a female guppy -
The adult male guppy on the otherhand, has longer fins, his body is sleeker and smaller than the female. The males will have colour on their top fin, body AND back tail. (gotta impress the girls :P) He also has a gonopodium (the boy bit) which delivers sperm to the female.
Check out this picture of a male guppy -
Guppies start reproducing at three months old. Imagine just how many babies they have in their liftetime when they breed every month (between 2-200 babies each month depending on the age of the guppy) and live to approx. three years old.No wonder they are nicknamed the millions fish!
Hasitha, if this doesn't help you stop bringing home only girls or only boys, Honey, you need glasses :P Love ya!!! hehehe
Information and pics to help me out were from -