Today I could do a more thorough check of some small plastic tanks with Austrolebias which I had put in a wooden garden house.
The temperatures inside have gone from -6 C to + 30 C over the last few weeks, as sunshine during the day heats the garden house very much. During summer, I have a solar-powered ventilator at work, but I had turned it off during the frost period. The total volume of water in the tanks inside was approximately 150 liter. None of the plastic tanks have frozen over at any point.
So the numbers are
Austrolebias bellotti Maschwitz 3 alive (2 male, 1 female)
Austrolebias nigripinnis Maschwitz 1 alive (male) 2 dead (male, female)
Austrolebias luteoflammulatus La Pedrera 1 alive, 1 dead (females)
which is not unusually bad for old fish over a period of two weeks.
What remains to be checked in the greenhouse, once all ice has gone, is whether 3 Austrolebias elongatus have made it.