Minnows are tough to breed, and comets need a lot of space.
Your best bet would be some type of livebearer...guppies, platies, mollies. Pretty much all you have to do is add water and you have babies

. Get a bunch of females, maybe one male, and you should start seeing babies very soon. If you have enough space/another tank you can let mom give birth in a growout tank, and then remove mom and let the babies grow. You can also leave them all in one tank (some babies might be eaten, but enough plants and hiding spots some will survive).
A lot of people breed convicts as well as feeders...as long as you have a pair, they will reproduce like crazy and do all the rearing on their own.