I must respond to this thread. We have many new members, meaning new to the hobby as well as the forum, and reading this thread they could easily get the wrong idea so the record must be set straight.
Putting fish in a new tank with no method of seeding the nitrifying bacteria is cruel to the fish. Regardless of how many water changes you do, they will be impacted, and this is permanent.
Crispin mentioned a case where out of necessity this had to be done, and that is understandable. When we have such situations not of our own making, we do the best we can. But never forget that high ammonia and high nitrite, however sporadic it may be through multiple water changes, is still damaging the fish internally, and this is irreversible. The fish are permanently injured physiologically. They will be weakened and thus more susceptible to other health problems all their lives, directly caused by this initial poisoning. I can guarantee they will live shorter lifespans, again directly the result of this internal damage.
Fish can be added to a new tank only if you have some means of dealing with the ammonia, then the nitrite. Live plants that are fast growing (floating are best for this) can allow this to work without damage to fish, other things being equal. Daily substantial partial water changes using a conditioner that targets ammonia and nitrite (Prime, or Ultimate) helps too. And seeding the tank with live bacteria or a bacterial supplement.
This was absolutely terrible advice from the fish store. And what about the deliberate harm to the poor goldfish? Using any fish to "cycle" a tank is cruel. This store's advice is something you should never rely on again.