completely fill coolant up almost everyday

Car had no leaks until about a week ago, where i have had to completely fill coolant up almost everyday. What could be wrong and are additives ok?

The car has had no leaks since i got it, but starting about a week ago, the coolant level indicator comes on and when i check, the reservoir is completely empty except for a little puddle in the bottom.

I have had to fill it back up every day. The temp gauge hasn't gone above half either. What could be wrong or what should i do? Is it ok to put something in such as a stop-leak?