@kevinsky mosquito fish are also native to Guatemala Gambusia nicaraguensis, Gambusia sexradiata, and Gambusia yucatana. (source)

@Eyal It is likely the cold and probably the fog. The edge of some of your new leaves look like they might have had some burns too. I would actually not worry too much they do this every time it gets around 3-5C and lower. If it gets too cold below 1.6C (35F) they may drop their leaves and new stems can turn black and die.