If you have some skill, breed apistogramma. They arent the easiest out there but are usually semi valuable and comfortable in a small tank and look gorgeous
Kribs are brilliant species to breed I've had quite a few breeding pairs over the years , easy to sex them breed like fun and great parents , there colours are stunning especially when spawning and protecting young . if you can find some super red ones even more impressive colours on display , they cost a bit more but are worth it as kribs are quite cheap fish in first place
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Australoheros red ceibal will breed at very small size, 3" for male 2" for females.
A 20 gal may work for a year, but a 10 gal is pushing smallness, they will outgrow a 20 gal at about a year though reaching about 5" to 7".
They don't need a heater, and are generally very hardy.
They have spawned for me in algae choked outdoor tanks, and ponds spring thru fall in Wisconsin.