Please make a list of what you see. What are your water parameters which you keep an eye on? Are the fish disfigured in any way? Are their fins ok? Are their any marks or discoloration, on their bodies.  Are any discolored or hollow bodied?  Are they breathing or swimming any differently?

How long have you had the guppies? What did you do  in setting up their tank and getting them settled? It sounds like you must have several, if there is one to lose every day.

What kind of water are you adding salt to? If you have very soft water, adding salt (unless it is a marine salt) is dangerous because of the perponderance of sodium to the exclusion of all other important minerals. Maybe I'm totally wrong, but take a look at Winter Mystery Deaths, under Disease Prevention, via the Quick Links on the upper right hand corner of the GL homepage.

Please also look at the other disease write ups there. Hopefully there is a discussion which might ring a bell.

Although this sounds a bit goofy, if you have some seasoned water, pick up the number of partial water changes. If the new water is good for them, it can't hurt them. I'm sure cichlid people, probably (of necessity) are the most faithful of water changers and may make that same suggestion. Unless there is a specific medical need, don't any more add salt.