The least killifish is misnamed. The problem is with the killifish part–it isn’t a killifish. It is a livebearer, similar to guppies and mollies. It is the smallest livebearer in North America. The genus/species for this fish is Heterandria formosa.
These live in a five gallon planted tank that s plenty big for these guys.
I was unable to find, much less get a photo of the tiny male. The image below is of the female.
The thermometer is probably a centimeter wide, so that should give some scale to the fish behind it. Click an image for a larger photo.
