They do not like salt in their water, first off. I also abhor the use of salt as a 'prevent all' for all fish - because it isn't. It can do more damage then good to many fish. If there is a problem with some fish, find the problem, don't just 'add salt.'
Also, I am familiar with aquatic plants, there is nothing toxic in their tank. I would never do <i>anything</i> as irresponsible as adding toxic plants to an aquarium! And yes, we've tried them in bare tanks too, and other kinds of set-ups as well.
We've established the need for something to help their slime coat, see above posts.
We've nailed it down to the fact that the fish are being mistreated somewhere along the lines before they get to us. Probably, and unfortunately, by our suppliers. >
We did another order, and we managed to save a good portion of the fish this time around.