All living organisms such as fish,plants and animals need water to survive and also the require certain pH level of water in order to be healthy.Its a acid and base balance reaction.Strong acids dissolve into water and its changes water's pH.if water's pH level drops it becomes acidic suppose a fish waste ammonia releases and dissolves in water to form harmless ammonia ions,carbon dioxide ions and hydrogen ions even some limestone also rises pH level and water becomes more basic.To make the tank water alkaline, add some corals and aerate the tank water by fixing an air pump so that excess of carbon dioxide eliminate out and water pH level comes to neutral this is the reason water becomes slightly alkaline and the fishes are able to reproduce.
