Livebearers & Fry
There are several things you can do that will encourage healthy fish and fry. Large, frequent water changes of at least half the volume of the tank and done weekly. Feed a variety of foods. Mostly frozen foods of a good brand like San Fran Bay. Freeze dried tubifex from Hakari or Tetra and flaked from Cobalt. Feed a little every day and more often when you see fry in the tank. Well fed adult livebearers are less likely to bother the fry. Heavily planting the tank with floating plants like Hornwort will make the fish feel comfortable and secure. I would add a bit of standard aquarium salt to the new, treated tap water, 1 to 1 and a half teaspoons in every 5 gallons will do. This is likely the maximum. Most freshwater aquarium plants don't deal well with salty water.
"Fear not, my young apprentice. Just change the tank water."