Its kinda tough to get ahold of confirmed male and female adult cons w/o growing them out so i would go with Mollys, Platys, Guppys or any Livebearer if your a beginner.
They will give birth to live young, that must be seperated or the parents will eat them up. A birth box is made for this thats sold at most LFS. Depending on the size of your current stock should effect what you choose, smaller fish will be fine w/ fancy guppys and you can grow out A LOT of them in a 30. If you have larger fish i would go w/ platy or mollies. Jeff rapps has some nicauraguan mollies that obtain lengths of 5 inches if your interested in breeding those.
I cant wait to get my own breeder thing going again, had a palmas polli bichir in there to make sure he wasnt sick or anything (i basically use my breeder as a QT, as my breeding fish usually are cheap and small i dont mind them being picked off) had it divided but the lil sucker somehow got to the other side and picked off most of my breeding stock, so now i have a butterfly fish in there because i thought my TATF would tear him up
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