how do fish mate?
Depends. Livebearers, like guppies, mollies, swordtails, ect, mate like people do, actually, and give birth the same way, though guppies can actually be considered in between, because they drop the babies as they're breaking out of the egg, but are still encased.
Egg layers, like goldfish, neons, and zebra dainos, chase each other, the female dropping her eggs and the male saturating the water with a ridiculous amount of semen, fertilizing them.
NO fish do not have sex, My 2 angel fish has like a million babies... the female lays eggs on something, then the male comes a puts a bunch of sperm on the eggs.
well they all mate by having sex
then they give birth to either live young or lay eggs