It must be the chloramines, then. I use Prime dechlorinator and I go through a 250 ml bottle in about a month and a half (I have a 55 gallon and a 5 gallon tank). I just did the tap water test and got a slightly higher result (1 ppm) . Our water authority does add both chlorine and chloramines to our water, in such large quantities that our LFS recommends double dosing dechlorinator, particularly after heavy rains. I will not worry about it unless the ammonia level does not drop to zero in 24 hours. Thanks for the help.