How tiny? Sounds more like planaria to me. Planaria is a natural inhabitant of fish tanks and it's possible they were either already in the tank and got attracted to the plants, or they moved in with the plants from the garden centre.
Planaria come in all sizes, ranging from the size of a pinpoint to several millimeters long. I've seen them as long as a centimeter in my breeding tanks. Made great live food for the Dad

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Males: Sparky and Marble-Valentino(VT); Blue Tide, Smoochie, Marble Boy (SD), Dimer, Eragon, Tiger Tail, Watermelon, Freedom, Sirius, Draco, Magicarp, Black Dragon(HM); Kitkat (PK)
Females: Copper, Bully, Parvati, Blondie, Bella (HM).
Current fry: Copper/Orange HM, Full mask blue HM, Blue dragon x gold dragon HM