Spaceman is probably right, your tank might not have fully cycled yet...
to answer your salt question...once you have added salt to treat the entire tank, only add salt after removing tank water.
Example:
you add salt to treat your 55gallons
a week later 5 gallons has evaporated from the tank so you add 5 more to keep it topped off, do not add salt to this 5gallons you are adding.
when you do water changes, only add salt for the amount removed (if you remove 20g and add 25g, add salt for 20gallons worth).
At least I'm pretty sure thats how it goes, someone correct me if not.