The best foods for conditioning rainbowfish and livebearers is Frozen: Marine Mix (prawn, fish and squid), raw or cooked prawn/ shrimp, brineshrimp, bloodworms; and live mozzie larvae and newly hatched brineshrimp,
Feed the fish 3-5 times per day for at least 2 weeks before breeding.
Do big water changes and gravel clean the substrate every day or two while feeding more often so the tank stays clean.
Deworm the fish before breeding them. Most common livebearers (guppies, platies, swordtails & mollies) that come out of Asia are full of intestinal worms.