Arsenic is probably the problem. You might do better off getting american raise and bred guppies. I know that some importeted fish have been give arsenick so the systems statr breaking down and the survival rate is low so peopl have to go back an buy more. I buy fish from pet stores not expecting the adults to live long. I exppect to get the fry and work with them. Chales at Goliad Farms and I have spoke about this and we agree that this is what is happening. He really know his stuff. I have never see this with guppies but highly possable.
You might do best set up a few tanks in the back of your store and raise you own guppies or keep in contact with local breeders in your area and sell american bred and raised. Check you local craigs list and other resources and see if you have better luck. When I grew up and raised guppies I never added salt. People now seem to thin adding salt is the cure all. I believe more frequent waterr changes is more effective. There was this same myth about mollies. I learned frequent partial water changes did more. A little salt may jut be all you need. You will have to find out what works for you.