I wonder if you guys ever noticed or heard about this little trick swordtails use to catch fry.
Today, after one of the females gave birth inside the tank ('cos they never let me see when they drop and I never get the chance to move them to a breeding net), while the little guys were hiding in the gravel, I noticed more than once some of the adults move violently near the bottom, creating vacuum that sucks up the dirt from underneath the gravel, obviously, to uncover fry that's eventually hiding down there...
BTW, it never worked, and I got to rescue 21 gorgeous little fellows...

Can't say as that I've ever seen them do that. But then, I don't keep gravel large enough for the fry to get into it in my tanks.

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I have seen them dart, sort of short burst darting, along the bottom like I think you're talking about. Get 10-12 doing that and they become an extremely efficient killing machine.

I must confirm my observation. They are doing it on purpose. But I must say I didn't see all the fish do that, it seems that every swordtail has a personality on it's own. On the other hand, whenever I see the fish display some sort of strange behavior it's almost 100% sure there's fry in the tank. They all find fry delicious

My swordtails do the same thing, only they do it the opposite way, i.e. they make the same motions to suck out fry that hide in my frogbit plants which float on the surface. sneaky bastards.

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• Tanks: 1.8g Tetra Waterfall Globe, Aqueon 10g 20x10x12", & Marineland 10g Half Moon
• Plants: Anubias barteri & A. lanceolatta, Amazon & Brazilian Swords, Java Ferns & Moss, Water Wisteria, Water Primrose, Banana Plants, Frogbit, Moss ball
• Fishies: Red VT B. splendens, Blue-Green Dragon PK B. splendens, Mustard Gas Monster HMPK B. splendens, Swordtails x4, Platties/Variatus x6, 1 Molly, Dwarf Cory Cats x3
• Inverts: Zebra Snails x3, 1 Ghost Shrimp