If it's only livebearers in the tank, you might want to go straight to the heavier dose aquarium salt treatment. He's likely damaged his slime coat at the very least, possibly scales and fins, and that leaves him open to bacterial and fungal infections, especially with the added stress of being new to a tank and possible hassle from other tank mates. Salt has antibacteria and anti fungal properties, and could help prevent an infection setting in.
I had a female guppy fall from the tank once when I was removing the sponge filter, and it tore most of her tail off and damaged the slime coat down one side. She didn't even fall as far as the floor, just from the tank lip to the stand, and I saw it so she was straight back in, but it's a nasty incident for a fish, a big trauma. She had a fungal infection by the next morning, but water changes and salt treatment cured it.
Day after injury, most of her tail gone, fungal infection setting in at base of tail, and along the other side that you can't see in this photo. Within two days of salt treatment the fungus was gone, and her tail regrew. She became my largest and healthiest female guppy.
