1077 has a good point, to rinse plants off before planting them.
The loss of leaves on newly-acquired plants can be due to a couple of things, both unavoidable. First, some plants are sensitive to any change in their environment; the species of Cryptocoryne are notorious for "melting" when there are changes in the water parameters or quality. They can withstand replanting better if the water is near identical; when I moved several years ago, I pulled the plants out of my 90g and 115g tanks, and laid them in a spare tank with enough water to just cover them. They remained in this tank for 2 days before I got the main tanks set up again after the move. To my surprise, the majority of crypts did not melt. I assume this was due to the water being identical in pH and hardness. A year later, the water board decided to raise the pH of the tap water from 6 to 7 and I did the usual weekly partial water change without knowing this. Within two days all the crypts in both tanks had melted into a pile of mush. And since then, I have had meltdowns result from any change in water parameters, including temperature. The crypt roots usually remain alive, and new leaves will generally appear, sometimes within a week or two, sometimes months. But other plants are generally not this sensitive unless the difference in hardness, pH or temperature is significant.
Some medications can affect plants, and usually loss of leaves is the result. I used Maracyn to combat columnaris a few months back, and after about a week I noticed the pygmy chain swords did a meltdown, and the large red-leaf swords also lost their leaves; the large green swords were less affected. They all bounced back within 3-4 weeks. But obviously something in the Maracyn affects plants. Another member on this forum had the same experience. Changing fertilizers, or changing the frequency, can affect some plants.
Lastly, it is common for plants bought from the fish store to lose their leaves if they are relatively new from the supplier. Many of the plants we have in our aquaria are not true aquatic plants but are bog plants in nature. Almost all the swords (Echinodorus species) are bog plants, and many of the crypts. In their natural habitat, they live in marsh or bog conditions, the roots constantly under water throughout the year but the leaves grow in the air (emersed) during the "dry" season; when the rains come the rivers in Amazonia and parts of SE Asia flood the forests and the plants are then submersed for six months. Most of these "amphibious" plants do quite well permanently submersed, although they will not flower unless thy have a period of emersed growth as they do in nature. The leaf forms are sometimes quite different between those that form above water (emersed) and those that form underwater (submersed), both in shape (the most obvious difference) and texture, and sometimes in colour. These aquatic plants are generally raised emersed, not submersed, because it is quicker and less expensive. Thus, when you plant them in the aquarium, the emersed leaves will die off and the new leaf growth will be the aquatic form and frequently quite different in appearance. This is obviously perfectly normal and not a cause for concern; the plant is simply adapting to the sumersed form, and it will remain in that form in the aquarium for many years. I have an Echinodorus macrophyllus in my 115g that is 12 years old, and I have four daughter plants from the last flower spike it produced in May.
Byron.