What sort of plants will you keep? If they're root feeders, regular root tabs will work for you.
The tank is probably too small for a Puffer and maybe too small for Pygmy Cories - but they'd be more likely to be okay in the tank. The only truly suitable livestock is shrimp. Maybe you could add a bunch of Cherry Shrimp and maybe some Otos?
I'm hesitant to recommend anybody keep Dario Dario in a tiny tank. Since they're small and territorial, it's tough for them to make their own space in a tank with that footprint.