In between water changes, here is some very useful material to check out
Fish in cycling
Keeping the fish safe is more important than worrying how much the BB is or isn't growing. BB will continue to grow and it might be a bit more slowly than if in a fishless cycle.
Really clean water will benefit your fish.
This fish / GBR is a type which is better placed into the tank after the tank is stable and matured, as they can be sensitive to poor water quality. It isn't a first choice for a brand new tank.
It is often that information from the lfs
is not beneficial to the life of the fish. Very often because the employees do not know, and do not understand how the nitrification cycle really works. The nitrification cycle information is one of the top pieces of information you need to be successful with your tank.
Although you have 0 NitrItes and Nitrates which is just fine, ammonia can cause stress, as well as fluctuating water quality which happens when doing a fish in cycle of a tank.
Definitely do not feel like the information is being put out there to be mean. Personally I want you to have way more information than you need so you can make solid decisions and have a happy tank you love. Losing fish is sad and hard and expensive and makes the fish hobby not very enjoyable.
Keeping happy fish in the right conditions and in a smoothly running tank is fun very enjoyable.
So read all you can and do plenty of testing and water changes. You will hopefully save this fish, and keep your tank mates safe during the fish in cycle!