What happens if fish tank water is too alkaline?

Water that is too alkaline harms fish growth and health. When pH is above 7.5, some minerals dissolve poorly, reducing the fish’s mineral uptake. Alkaline water also corrodes skin and gills, causing swelling, ulcers, and abnormal gill breathing, even gill filament rot.

Finally, pH that is too high or rises too fast converts ammonia nitrogen into molecular ammonia, increasing water toxicity and potentially causing large die-offs.