My three A. elongatus which were outdoor didn’t make it through the frost.
Looking at the survivors of the cold period in January-February, it seems that the survivors are restricted to part of the Austrolebias phylogeny I made based on molecular data. This part includes some relatively large species. The amount of data is, of course, almost negligible, so this is just an impression leading to a new hypothesis:
Large size in Austrolebias might be in some cases an adaptation to cold environments.

Build it solar

How to avoid tanks from freezing over in case they have to be left in the garden? The Build it Solar website lists many examples of solar heaters and other DIY projects. Particularly inspiring examples are the Solar Horse Tanks.
On the other hand, inside a wooden barn which is a bit better isolated than most greenhouses, it is maybe sufficient to have a large water mass for heat storage, for example by stacking a number of plastic tanks with water in a spot which receives much sunlight during the day, not too far from the tanks with fish.
Should be ready by next winter…

Freezing cold

Europe is suffering under a cold spell. Austrolebias don’t necessarily die when it freezes. I’ve filled a small greenhouse with 90 liter tanks, some containing few old fish bred in 2010 and early in 2011. These fish are too old and too large to transport to a different site and have already survived a cold spell outdoors, with 3 cm of ice on their tanks. They are otherwise healthy. I want to give them a chance to reproduce in spring and therefore decided not to disturb them nor euthanize them.
In the greenhouse, during the night, tanks have so far frozen over with ice of about 1 cm thick. Since cold weather usually means sunshine, the greenhouse heats up during the day, removing the ice cover again and allowing the fish to breathe well. The total amount of water in the greenhouse is large, about 400 liter. I hope everything goes well, but there is a risk of failure. Then I will need to invent a better system in order to protect my old fish; or move or euthanize them all before winter.