I'm stating this generally for egg scatterers, but I do think it's the case for serpaes as well.
When she lays eggs it will be when she's darting around throught the 'bushes' with the male. Generally this happens at night closer to dawn. They'll both swim around and release, and the eggs and sperm will mix and fall to the bottom. The parents WILL eat their own eggs. I'm not sure about plecos, but I really wouldn't put it past them. If you have enough cover, you might see a couple survive. Otherwise, if you want more, you can move the pair to a bare-bottom tank filled with half fresh and half tank water and packed with mops next time it storms really good. The pressure change seems to be a trigger. Put them back in the main tank the next day and after a little while you may see fry start to pop up in your breeder tank.