Guppies. I've been unsuccessfully trying to breed forever, but that's because I don't care enough to get them a breeder tank.
They don't need anything really.. Temperature barely matters, pH barely matters.. Water quality barely matters.. Food barely matters, stress barely matters, etc, etc..
They are usually stagnant water dwellers used to eating mosquito larvae faster than they can breathe.
Just get them a tank, any filtration will do, keep the water 70F-80F. Plants don't matter, plastic plants and anything really for hiding works. Try to keep one male to three females. *Surface shelter is important* The fry will be born ready to swim, but will remain at the waters surface for the beginning. They can eat crushed flakes immediately, and almost no care is necessary. A female guppy can give birth to 2-200(30-60avg) guppies a month, based on size of mother. She can hold semen for the future so once a male mates with his female, she can be removed for two months.
Good luck breeding these things, I hate them honestly. Keep all the females you recieve and feed the males to your fish.
Males are distinguished by several factors;
Gonopodium (anal fin is thin, flexible, and reminds you of a refueler plane)
Bright colors, usually the first generation gets the basic "eye spots" towards front and back, and the later generations will develop more colors. Female color is rare.
Females get about three times larger than males, and usualyl have a bent spine with a huge stomach.
Have fun, I hate guppies.