How do fish mate?

Answer 1

It depends on the fish. Most fish are egglayers, where the female lays the eggs and the male fertilizes them externally. Fish like mollies, guppies and platys are livebearers, where the male internally fertilizes the eggs and the female gives live birth. Fish like chilids are mouthbrooders, where they keep the eggs in their mouths until they hatch, and protect their young by using their mouths. Gouramis make bubble nests, laying their eggs in a nest of bubbles at the surface of the water. There are many ways fish mate, it just depends on the fish.

Answer 2

It depends on the fish. All fish produce eggs, but sometimes the eggs develop inside the mother, and sometimes they develop outside of her. The most common livebreeders are the aquarium variety, the guppies, mollies, platies and swordtails. The male has a penis-like organ called a gonopodium (essentially a modified anal fin) that he uses to place his sperm inside the female's vent. They swim up her oviduct and fertilise the eggs that are inside her. She gives birth to live young when they hatch three to six weeks later. Most other fish the female deposits the eggs (in the gravel, in a floating nest, on an ornament, on plants, it varies widely) and the male sprays sperm over them to fertilise them. Sometimes one or both fish will stay to guard the eggs and/or fry.
Normally (I havent heard of any fish that mate any different), the female will lay her eggs and the male will fertilise them with his sperm :)