It must also be monitored. Aim for 0 ppm. Anything above 0.15 ppm is stressful to the fish and can cause disease. To correct high nitrite levels, increase water changes and filtration, decrease feeding and stocking densities. Aim for ph of 7-9.
Fish can survive at a lower ph.
However, at this point, you are dangerously close to a ph crash, resulting in severe stress to your fish. Therefore it's important to continually monitor your ph. A ph above 9 is not harmful by itself. Lowering ph is a little more difficult. Adding peat or vinegar is somewhat effective.