API Aquarium Salt is a highly recommended item by The Betta Sanctuary. This is not the same as table salt (DO NOT put table salt in an aquarium).  It is used to help with fin growth and gill health and can be added whenever water is changed. Dosage is 1 Tbsp/ 5 Gal 
 
WARNING: Do not overdose because bettas are sensitive to aquarium salt additions. Only add the amount of salt neccessary for the amount of water that has been added because API aquarium salt does not filter out of the water.