Is
nitrate .3mg/l or 3mg/l?
How about nitrite (NO2)?
Is there a reason for pH being so high for type of fish you have?
In simple language, the difference of Concentration of Hydrogen ion (H+)for each number is 10 fold. Although some out many of one type of fish can tolerate/adapt to such extreme pH, at least we dont have worry/concern about wether pH caused death of fish if given proper pH to begin with.
Maybe your tap water is high in pH? or could it be type of gravel/deco which is causing pH to be alkaline (basic)?
If nitrate is up as you said, maybe due to product you added.
Perform test on tap water and and do another one w/ tap+drop of additive to see if the reading are due to additives.
You could same test for Ammonia (
NH3) and NO2.
Post the results so have understanding of your tank!