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Actually, there's a chance your female platy will end up giving birth to platy/guppy hybrids... Puppies, I suppose. Might be kind of cool to see...
Most livebearers seem to prefer larger females, which will naturally have more babies, so that could be why your guppy is after the platies.
I hate hearing people say "it's only a $3/$5/$1 fish/shrimp, so it's ok if it dies, I can just get another." It's still an animal! All animals should be treated like they're worth $10,000.
29 sw: Damsel, shrimpgoby, pistol shrimp, waspfish
65 fw: Rummies, glowlight tetras, pencilfish, darters, ottos, f betta, goby, dwarf gourami, ninjas
29 fw: Chili rasboras, pygmy cories, P. Gertrudae