get a boy and a girl mature, add water, filtration, heat, maybe a rock. ur done!
if your breeding them for something to eat your gonna need another tank, parents will defend the fry, unless it were a 200g tank and fry eating fish were much larger and nastier than the con....
ive bred convits in a piranha tank in my younger days... a percentage of the fry survived....
not sure what your end goal is here...
guppies may be better if your looking for an all in 20g with something to eat the fry...
GL!