is there ammonia in tap water or any other substances?
They may add ammonia and chloine/hypochlorite to make the disinfectant chloramine, or jast add chloramine direct to tap water. This is also dangerous to fish, but cannot be removed by leaving the water in an open tank for a few days, you apparently remove it by adding Na
2S
2O
5.
Also ClO
2 may be added to your tap water, there is a small list of disinfectants used
here about two thirds down the page. There are other chemicals used in the water purification process that you might want to look at.