ok that's good, no nitrites in the tap water
ammonia and nitrates are the things most commonly found occasionally in tap water
Don't know -why- (especially after 3 50% changes! done fairly close in time?) nitrite is not being diluted but unfortunately, the goal doesn't change. Nitrite(NO2) needs to not ever go over 0.25ppm if at all possible.
good luck, must run,
~~waterdrop~~