Your tank is cycling. You have been given some great advise above I suggest you follow it. New tank syndrome/a cycling tank is very dangerous for your fish. The high ammonia and nitrite concentrations in your tank could cause burns to gills, deformities and even death. Even after the tank is cycled if the fish have been exposed to too much they can have on going health issues and can die prematurely.
Do not use pH altering/stabilizing chemicals. They can poison your fish and cause large pH swings that can stress and kill fish. They will adapt to the pH of your tap water. Stressed fish have lowered immune systems. Lowered immune systems mean disease can get a foot hold.