high NitrITE in a new aquarium mean one thing. your aquarium is not cycled. I agree with everything said here.
As Neilanh said, a cycle takes longer if you have fish in your tank. Unfortuantelty the bad news for you is that the path to a happy tank will involve daily water changes until the cycle is complete.
You need to know three things at all times. 1> your ammonia levels 2> your nitrITE levels and 3> your nitrATE levels.
Ammonia and NitrITE need to be kept <= 1ppm (pref 0.5ppm). If they go higher, perform a partial water change (PWC
). Personally, you seem to have very high levels that are extremely toxic to your fish. I'd change 50% and another 25% in around 10 hours. Re-test and go from there.
Keep adding Prime to the fresh water and drop usig the Cycle.
Taking the established gravel from someone elses tank is a good thing and will help. And take heart in the fact that you have come to the right place looking for answers. Be wary of what LFSs tell you and check here first. We were all beginners once.
That Petco guy was right in the sense that good bacteria will grow more slowly the more watger changes you do during the cycle, but he was wrong to tell you to only change out 20% of the water when you have fish in it.
Cycle with fish = as many PWCs as it takes to keep levels as stated above. Anything less will result in dead fish.