The water cycle describes the sequence of evaporation and condensation of water in our environment. It is the sun which provides the energy to initiate the cycle. Evaporation from lakes, rivers and the sea carries water vapor into the atmosphere where it cools (condenses) to form clouds of water droplets. Once the droplets reach the ground as rain, they percolate through the soil and gather to form rivers, lakes and ultimately oceans once again.

As water travels around the cycle an number of factors influence its chemical composition. As rain falls, it can absorb oxygen, carbon dioxide and other gases such as sulphur dioxide emitted from power stations producing "acid rain", and during thunderstorms rain absorbs nitrogen oxides produced by lightning.

Arguably it is once the rain reaches the ground that the greatest chemical changes occur. As the water filters through the ground, it becomes charged with carbon dioxide from the respiration of plant roots and other organisms. It is usually this that makes water slightly acidic. Depending on the ground, other substances will dissolve in the water. Water drained from farmland will contain nitrates from the effects of fertilizers, filtration through chalk or limestone causes water to become alkaline and hard and water which has collected underground is usually high in minerals.

The terrain through which water passes before finally reaching the rivers greatly influences the water chemistry and thus the conditions in which our fish thrive. It is through our water changes and filtration methods that the aquarist tries to replicate stages in the water cycle to create ideal conditions for the fish.