If there's no ammonia in your tap water, the source of the ammonia in the tank is the fish. Which means for some reason, the filter's not converting the ammonia anymore. 100% water change will get rid of it, but the minute you put the fish in, they start producing ammonia again so you need to move some established filter media over to this tank. No need to clean everything thoroughly, the ammonia is in the water not on the sides of the tank and gravel. You're not being rude at all. I'd be 100X more stressed than you if this was happening to me!