You can't test for ammonia if you have an ammonia-detox bag in your filter. It tricks the test kits. ....looks like its off the charts when actually its the harmless bound ammonia.
Sharpie....guess I was interested (thinking) that the tank had been previously used for something like amphibs or reptiles. I have known of people trying to use those used tanks for aquariums...most got limited sucess. I remember more than one complaining of a "urine smell". Dangerdoll brings up a good point...if you previously had fish in it, then you are starting from scratch, effectively. Hurricane also has a great point on what was used to clean the tank, you might have some residual cleaning agents involved.
i just used hot hot fresh water. and a little half capful of bleach, but after i did that i rinsed the tank out like crazy and then i let it sit out in the sun and dry......(was this the right thing to do)....i just used hot fresh water to clean the rocks and fake plants....