Some one-inch fish:
Dwarf livebearer
Neon Tetra
White Cloud Mountain Minnow
Pygmy Corydoras
Harlequin rasbora
Serpae Tetra
Dwarf pencilfish
Of those, I would say that White Cloud Mountain Minnows and Harlequin Rasboras are truly hardy and easy to care for, especially in a tank that's at least 15 gallons. Serpae Tetras, pygmy corys, and dwarf livebearers are also pretty easy. Neons are prone to shock and disease, but are actually pretty hardy otherwise; you just need to quarantine them and introduce them slowly to a tank (using the drip method). I have never had dwarf pencilfish, and accounts of them vary.
