What fish would work for Walstad low/no tech nano?

White cloud minnows

Least killifish

For such a small tank, I'd go with the killifish not the white clouds.

You could have a good size group of least killis, white clouds get a little too big for nanos.

White clouds and CPD's need room to swim. What about some of the smaller rasboras? Harlequins, chilis, or emeralds?

Also, I have some Endlers that are doing fine in a 2.5G. It has a small heater, but turned down low around 72F. What's your room/water temp?

Least Killies (which are actually a US native livebearer) are the smallest livebearing fish in the world. I've got a colony in a 5 that's been chugging along. Tons of personality, hardy as frakking heck (I had 11 surviving in 40 degree water last fall before I was able to get them out) and would be ok in small spaces. Males are smaller than females, and have some very entertaining territorial behavior if it's a boys-only tank. Males and females spar on and off, but no one gets hurt. I think of them as the fish version of chihuahuas.

Only thing I've noticed that could be of concern is that males seem to assume that all other fish are both female and willing partners, regardless of species or sex. My male scarlet badis were not pleased.