It is really important to understand that we only test a teensy-tiny number of parameters of those that actually may impact animal health. These are all changing over time as the tank matures - and as your skills as a hobbyist mature. Just because the "big 7" are tested (amm, nitrite, nitrate, pH, alk, Calcium, specific gravity) and within norms, does not mean the tank is ready for some of the more delicate forms.
I would also mention to be careful of stocking with fish too quickly.