Alkalinity is the measurement of how basic or alkaline water is. Water with a pH greater than 7 is alkaline, as opposed to acidic. Every species of fish has adapted to a specific set of water conditions, including pH. Keeping a fish in the wrong pH can stress and kill it.
Chemical Stress
Keeping a fish in dangerously alkaline water can have a number of serious health effects on it. Stress leaves aquarium fish vulnerable to disease, degrading their health. Additionally, sudden changes in alkalinity can cause fish to lose control over their swim bladders, making it hard for them to swim correctly. Lastly, raising the alkalinity of aquarium water transforms ammonia -- found in fish waste -- into a more toxic form that can poison fish.
Reducing pH With Reverse Osmosis Water
In the wild, water with low alkalinity always has low hardness. So adding acids to an aquarium might lower the pH out of the alkaline range, but this water won't resemble anything a fish would encounter in the wild. Instead, you can lower the pH and hardness by using water purified by reverse osmosis. This process removes almost all of the dissolved minerals in that water. You can add reverse osmosis water to aquarium water to dilute it, which lowers the alkaline and hardness.
Lowering Alkalinity With Decorations
You can also lower the alkalinity with certain aquarium decorations. You can add oak leaves, which will slowly release tannins into the water, lowering the pH. Malaysian driftwood and coconuts have a similar effect. Again, this works best in soft water, such as reverse osmosis water. These materials tend to release tannins slowly, ensuring a stable pH.
Guidelines
Whenever you change aquarium water chemistry, remember that sudden changes in water parameters -- even toward optimal conditions for the species -- stress fish out. To avoid this, never lower the alkalinity of aquarium water by more than .2 pH units per day.