Hello Bob...
I don't know if you can make your "Livebearer" physically stronger, but you can definitely make it healthier. Feed a variety of frozen foods, like Bloodworms, Krill, Plankton, Mysis and Brine Shrimp. Look for those with no additives. All you want to feed is the frozen animal and added moisture (water). I recommend feeding minced garlic once a week too. Just garlic, nothing else added. Garlic has a long list of vitamins and is a natural antibiotic.
Additives tend to be things that fish don't digest well. Don't use foods with a lengthy list of ingredients. If the ingredient isn't familiar, I don't give it to my fish. Pretty simple.
Just a thought or two, you're the "Fishkeeper".
B