You may not get an objective answer from the guy at the fish store, you may not get the answer you need to assist you to moving in the right direction with your tank. Beware of products claiming to be the answer that he may try to persuade you do buy.
If this guy knew you had a 6 gallon tank and did not suggest that a Scooter Blenny, or 3 fish in that size tank would end in disaster than you might want look elsewhere for guidance. His motives might have been to sell you live stock and products rather then what is best for your tank and your livestock. Just my 2 cents.