What type of breeder box are you using? Many imped water flow and the resulting water quality in the box is harmful. You may want to get another 10gal to use as a rearing tank. The fry will grow much faster and if you use live baby brine shrimp will be big enough to place with the parents in 2-3 weeks. When I bred guppys I used a 10gal with water, heater, sponge filter rated for 40gal and that was it. I raised the fry in these tanks as you can siphon the excess food and waste without sucking up fry and fry can typically find and consume food much nore easily.