First, you need enough rock in your tank to house beneficial bacteria.1-2 lbs per gallon.
Second, you need an ammonia source for the bacteria colonies to grow. This is part of the nitrogen cycle also known as cycling a tank.
Third, add fish slowly. One or two at a time only and allow 3-4 weeks before adding more. This allows time for bacteria colonies to adjust(increase) to the added bio load.
Last, don't trust LFS
. They are there to make money!