Fulica prefer over 21 degrees C, and the 10 is definitely too low. I recommend you set up heating for the tank, in the form a heat mat attached to the side (NOT bottom) of the tank, and also insulation board on the top, back and sides of the tank. Kitchen foil works surprisingly well as a temporary solution, if it's difficult to get to a hardware store for the insulation board. Just make sure the snails get both heating and insulation.
Fulica aren't awfully picky when it comes to what is the temperature right at the heat mat, they adapt to warmth better than they do to cold, just make sure that the heat mat itself isn't hot enough to burn (could you lean your bare hand on it and not burn, like, all day? If yes, it's not too hot).