Your tank is going to take time to cycle even with stability.
Since you have described as overstocking your tank, then I definitely would want the most efficient bio media that can fit in the limited space of your filtration.
Prime will detoxify the ammonia present in your tap water and convert to ammonium.
unless your using a test kit that can differentiate between Nh3 and Nh4 (like seachem), Most likely your reading total NH3 NH4 if using a kit like common API brand one.
What are your other readings such as nitrite and nitrate ?