I purchased a 20 gallon tank. At the moment I have 2 tetras out of 8. 6 died and I am unsure how.

Testing the pH, Nitrates, Nitrite, and Ammonia, everything seems normal. The other two live happily as ever though so I am assuming chemical wasnt the problem
Was it just that I introduced too many into the tank at once?

I am on hold right now before I add more fish. I am not satisfied with that gravel I have. I went with a the noob mistake of that tacky blue gravel instead of a black/natural.
I am waiting for my eco substrate to come in before I completely change that.

My aquarium has been running for about 1.5 weeks now. Again everything reads correctly. Temp range is 76-78.