Over a pH of 7.0, ammonium (the not so toxic form) converts to ammonia.
Other influences that make ammonia more toxic are temperature and pH.
You also get better bacteria growth at higher pH's. At pH's lower than
7.0, ammonia is converted to ammonium but the lower the pH goes the
less active the bacteria become.

So, your pH of 7.2 means there is more toxic ammonia than non-toxic
ammonium. Your test kit probably has some way to calculate the real
amount of ammonia versus ammonium. I know my Tetra kit does, and it's
temperature and pH dependent.

For sake of argument we'll use a total reading of 3.0 (not an
unreasonable number in cycling tanks), your pH of 7.2, and an assumed
temperature of 72F (because that's as low as my Tetra chart goes ;-).
The factor that applies in this case is .0072. Thus, the reading of 3,
times the factor of .0072, gives you a calculated ammonia reading of
0.0216. While any ammonia is bad, there is a certain threshold level
that is really bad. On the back of my kit it says "Calculated levels
below 0.05ppm are safe." If you were to change the pH to 6.6 with the
same temperature and ammonia reading parameters, the calculated level
is 0.003, much less ammonia and more ammonium. Does this make it
somewhat clearer?

Also, be careful with those ammonia locking concoctions. I know for a
fact that the old AmQuel if you use it willy-nilly will drop the pH
like a rock down to nearly untestable levels.

Lilly