You will also want to keep an eye on your nitrates. If your fish don't seem to be suffering then you're probably seeing total ammonia (which was mentioned) so most likely more of nh4 (ammonium, non-toxic) vs nh3 (toxic) making up your total ammonia reading. The total ammonia definitely should be dealt with because nh4 can be converted to nh3 under the correct conditions (like a rise in ph).