Did they give you numbers for ammonia, nitrite and nitrate? It is quite common for a store worker to say the results are fine when they are not fine.
To explain in simple terms:
Fish poop ammonia, but this is toxic to them. In a 'cycled' tank, there is a colony of bacteria which eats ammonia and poops nitrite. Unfortunately nitrite is also toxic to fish, but in a 'cycled' tank another colony of bacteria eat nitrite and poop nitrate, which is less toxic than the other two.
Cycling is the process of growing these bacteria and it takes several weeks. It can be done with fish in the tank to provide the ammonia food for the bacteria, but unless daily water changes are done to keep ammonia and later nitrite at zero, the fish can be harmed.
It is now recommended to add ammonia from a bottle before fish are put in the tank so the bacteria colonies have already grown when fish are bought. This is what SlapHppy7 meant when he asked if you cycled the tank.
if you didn't add ammonia to grow the bacteria, you are still doing a cycle with fish as three weeks isn't long enough to cycle a tank.
This is why we need to know numbers for ammonia and nitrite as any number other than zero is harmful to fish.
