Whiptail catfish and discus aren't big sellers here.
Rainbowfish and Corydoras can be sold.
If you want a breeding tank, ie: a tank with a group or pr of fish that breed, and where you can rear the fry (baby fish) up in the same tank, look at Apistogramma dwarf cichlids. Most other fish normally need to be separated from the eggs otherwise they eat them.
If you want to breed rainbowfish, put a pr in a tank with some Java Moss or a spawning mop and leave them there for a week, then remove the adults and rear up the fry in the tank. Or remove the spawning mop/ Java Moss and put it in a rearing tank or pond.
It's summer in Australia and you can keep and breed fish outdoors in ponds, storage containers or swimming pools. If you have space in the backyard, perhaps look for an above ground swimming pool about 6-10 feet in diameter and a couple of feet deep. Make sure kids can't get to the pool.
Put a pr or group of fish in the pool for a couple of weeks and then remove the adults. See what appears.