No, don't add salt during waterchange. They are mostly freshwater fish remember? To keep diseases and parasties out of your tank, always quarantine new fishes and plants before introducing them to the main tank. You can use medication, salt and otehr treatments in the quarantine tank. Some medications harm plants, so read up first.
There are many processed/manufactured fish foods out there that contain vitamins and such. You can also try some products like Dennerle's V-Max. Shop around the better stocked LFS for products. New products are constantly being added.
Generally, keeping your fishes in good health with regular water change, good waste maintenance and feeding routines (i.e. not over-fedding, clear uneaten food, etc) will help them keep their immunity levels up and make them less susceptible to illnesses.
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