Chances are your tap water is treated with chloramine. You should be able to find this out from your suppliers website, or by calling them. Some test kits will show chloramine as ammonia, and unfortunately this does not gas out like regular chlorine. Make sure that you use a dechlorinator that deals with chloramine and dose accordingly.but my tap water has 1ppm of ammonia in it.
Something like Seachem Prime would be a good idea until the filter is established as it deals with chloramine and de-toxifies ammonia and nitrite. But be aware that the de-tox effect is temporary (24-36 hours) so you would still need to do frequent water changes.