I've filled a new tank with water from my tap. When I tested it the ammonia was .5ppm. Does that sound right?

That was very good that you tested it, some times your water will have it in it depending on were you live. I have heard of this about others. You will have to moderate it

some water utilities use chloramines to disinfect the water... chloramines are prepared by mixing ammonia and free chlorine. What you are detecting is ammonia residual from the treatment plant.

i have a private water wail..i tested it for chlorine an it tested 0 so i stoped treating it..not long afteri began to have problems,,after not being able to drop the ammonia in my tank even after doing daily water changes i tested the water coming out of my tap..an it tested for 0 for chlorine an .25 to .50 ammonia started treating my water an have no problems once again,