How do you know when it is time for fish to mate?

Hey there, Depends on what what breed it is. Guppies, platies, mollies, swordtails breed all the time if the water hardness is right. Gourami will breed when a male is feeling like making a bubble nest which i think is normally spring and/or autum. Gold fish are normally in spring but need decent water conditions. And so forth. But If i didnt work at an aquarium for a few years I would have never known that most aqauriums only order in male gouramis and cichlids unless there has been a special order or the order is for mixed sexes, so u better make sure that u have different sexes first and sometimes it can be hard to tell. Do abit of research on the net or at the library and that should help u out. If u have any other questions u can message me at Hope that helped. catch!