Ammonia In Tap Water

Does your water company use chlorine or chloramine to disinfect the mains water? If they use chloramine, that is the source of the ammonia. But 0.5ppm is the allowed limit for drinking water whether they use chloramine or not.

Does your dechlorinator detoxify ammonia? If it does, that should make the ammonia non-toxic until the filter bacteria have had chance to remove it all. If your dechlorinator doesn't, I would switch brands.

Most of the dechlorinators on the market do detoxify ammonia, but not all. I use one that doesn't.
The one most often recommended on economy grounds is Seachem Prime. Available from some real shops and on-line shops but not Ebay or Amazon because Seachem refuses to let their products be sold there.
Others that do detoxify ammonia - Api Stress Coat; King British Safe Guard.
From what I can tell, dechlorinators made by Tetra, Interpet and Waterlife do not detoxify ammonia.

The detoxification effect wears off after about 24 hours. This is usually long enough for the filter bacteria to remove the ammonia in tap water.