Yes, that would come from either die off or what you seed it with ie. a shrimp. Best to test from the beginning. First you'll get zeros for ammonia, nitrite, and nitrate... then you will see say 1.0 ppm ammonia and zero nitrite and nitrate. So on until it converts to nitrate and you can filter with water change or skimming.
My cousin gave me some "used to be" live rock, but I am sure it is dead now because the empty tank has been in the garage for a while. Should I trash that and buy new live rock from a fish store?
PetSmart sell live sand, but it is in a bag laying on the shelf. I guess the bacteria will grow once we cycle the tank???