OK, so it's a trick - you don't really breed platies, just sit back and wait. I wanted to use the upper layers of the tank for a catch crop and with my hard tap water, I picked livebearers as the logical choice.
Swordtails take a long time to grow out to maturity.
I could have picked mollies, but again, the quality of the black mollies in shops is pretty low and I didn't want to put in the effort to raise good sailfins. Platies were the natural choice, and red wagtails are always a good store crop. It's important to keep a trio rather than a pair - even in a large heavily planted tank a male can be rough on a single female. The fish I chose had the clearest red bodies and darkest black fins I could find, without any black pigment on the body. I waited for the female to drop three batches of fry, then when I was reasonably sure that the fry being produced were fathered by the male I selected, I allowed a batch to grow out.