PAEDOCYPRIS PROGENETICA - Recently found to be the worlds smallest freshwater fish. Has an adult size of about 7.9 mm (~0.31").
Pygmy (Dwarf) Goby - Previous record holder for worlds smallest freshwater fish. Has an adult size of about 8.0 mm (~0.32").
I would bet you could place one of these in most of those aquariums. Of course...heating, filtering, feeding and a whole slew of other problems arise but hey, you can't have everything.
