Despite some pretty intesive searching, I cannot seem to find any
guidelines (and I'm not even looking for anything *definitive*), on
what constitutes "too high".

I'm about 10 days into a new 20-gal tank with 3 zebra danios. In
addition to keeping an eye on fish behavior, I monitor the pH and
nitrogen levels just about every day. NH3 levels are starting to rise
(still 1ppm) and NO2 is zero.

What NH3 value is considered "too high" where I should perform a water
change?

How about for NO2 (nitrates)?

Thanks in advance,

Mario