It has been the experience of some parties in Chicago, that when guppies and livebearers come in from the Far East, that their water has an amazing amount of salt. The shock of being moved rapidly to less mineral-laden water may be one reason why so many have been lost.

Your shop knows this and you probably do too, but for the benefit of others, adding a lot of sodium chloride will not show on any meters measuring DH or pH. A look at a hydrometer or TDS meter will indicate a staggering increase in the mineral content of the water.

Please see: Mollies, Salt and Asian Imports
http://www.guppylog.com/story/2004/1/31/5655/54195

I haven't done a systematic study of fish stomachs (nor will I ever), but I would also consider doing a preventive treatment with one of the commercial treatments for internal parasites. The percentage of fish with worms in the trade is just too high. Also, the active de-wormers in those treatments are a group of chemicals call anthelmintics. Perhaps it is dumb luck, but those anthelmintics are also sometimes useful against even hexamita and the much rarer Glugea. They are not antibiotics and one is not raising up bacteria with a resistance to antibiotics.