Maybe dumb question...can water quality be poor even if parameters are good?

I've had a couple issues these past few weeks. My Banggai Cardinal has fin rot, and I'm moving him into a QT today. However, I wanted to know if water quality can be poor and stressful in ways that don't show up on parameter tests? My levels are as follows:



Salinity: 1.025

Ammonia: 0

Nitrite: 0

Nitrate: 5-10

PH: 8.0



Any suggestions?