You need to prepare the tank for the fry. Livebearers require heavily planted surface areas for their fry. As soon as the fry are born they swim to the surface for a breath of air. If the surface is heavily planted, they're very likely to survive. If not, they become lunch for an adult. Hornwort is likely the best surface plant. The more single stems you drop into the tank, the higher the fry survival rate.
I keep large, heavily planted tanks of mostly Fancy Guppies and Platys, also fish that give birth to live young. When I see fry in the tank, I start to feed all my fish a little more and a little more often. Well fed adult livebearing fish are less likely to be interested in the fry.
"Fear not, my young apprentice. Just change the tank water."