ammonia in an empty aquarium.

I set up my 20gal (24"12"x16") aquarium and have been running it for a
week with no fish. Shortly after I set it up I tested the water and
found the ammoina level to be in the low detectable range .5-1ppm. And
it has stayed there for the past week. So my question is where did the
ammonia come from and what should I do to lower the ammonia? I
thought by now I'd have generated a bacteria culture and start seeing
the ammonia level go down.