Sometimes the weather is just not doing what you need. Recently hatched fry seem most sensitive to this, according to my impression. Some species seem to suffer more from cold nights than others. I’m using a small and relatively cheap Peltier incubator (Herp Nursery II) to keep some fry at relatively constant temperatures. What I do at the moment is raise the temperature to 25 degrees (C) during the day, and lower it to 20 when I go to bed. It seems to work well! Alevins of Austrolebias patriciae “Fontana”, aff. patriciae and paucisquama “Sao Sepe” all eat, grow and survive very well.