
Hi, and WELCOME to FISHLORE...
I would NEVER take the word of a fish store employee that the tank was "good to go", unless they tell you what the numbers are for Ammonia, nitrites, and nitrates. With the numbers, you can tell for yourself if it has completed the cycle.
If you don't have NITRATES between 5 &10 it ISN'T cycled. Also, the ammonia goes up, and nitrites will be down, until the bacteria develops to change the ammonia to nitrite...then the nitrite will soar, and the ammonia goes down....but until BOTH the ammonia and nitrite have been up, and then go down to 0, and the nitrate goes up, the tank isn't cycled.
Is this tank the same as in your signature?
The GBR in particular cannot live in water that hasn't been cycled. There are several issues with the fish that we can go into later, but for now you need to concentrate in waterchanges, and getting the tank and your fish through the cycle, so it is safe.
Did you use any product or method that introduced the right kind of bacteria to the tank to start the cycle?