hi yes you can add salt to your malawi tank, if required all it does is hardens the water if needed, i add salt to my tank i have a 140uk gal tank and i add 6 teaspoons of COOKING salt NOT table salt, and this does the job perfect giving me perfect water stats.
You can get special aquarium salt, which is just pure salt (NaCl) while table salt will have iodine and some other additives in there to keep it from lumping up. Note that aquarium salt is NOT reefsalt used in marine aquariums.
So whats the difference between cooking salt and table salt? I thought they are one and the same... Could use pickling salt though, as it does not contain the aditives found in table salt.