IMHO, I would use regular aquarium salt for them. Doc Wellfish's always works for me. 1 rounded TBSP per 5 gallons. The salt generally wont evaporate out, so only add salt when you do a water change- I usually add about 1 TBSP per half tank change when I do a monthly gravel vaccuum. However, for brackish conditions, youll want to use more- but Im not really sure how much...
A lot of people argue over whether or not to use regular salt or the synthetic stuff...