IMHO and experience.... the easiest fish you could breed in an aquarium are:
Bangi Cardinals (mouth-brooders)
They are sooo tuff.... I used to raise them in my overflows until they were big enough to sell/trade.
They would get sucked into my 2 overflows as babies, then I would just leave them in there. They would do just fine & grow fast with zero attention from me.
The 1'st time...I grew out' a dozen in my overflows before I even knew they were in there
