I've got 6 in my 20 gal too. I see spawning behavior when there is a change in water temperature. Usually when I do a water change with cold water. I also observed spawning behavior yesterday when the day temperature rose to well 29 deg C and then it rained outside. I have two big snails and a gourami which don't let their eggs hatch.
Good luck! Waiting for updates from you.
i find for many fish temperature changes will spur them. I actually started using the shock method for my bettas, because it yeilded a 98% chance of a spawn! My danios bred (so rare here for them to!) and I may try it with my guppies.