Water is a necessary element for all life forms. However, to fish it is necessary to their environment as well as their metabolic state. Fish not only process water through their alimentary and excretory tracts, but they also breathe it in with their gills, using it to gain necessary oxygen and expel carbon dioxide. There is also exchange of water metabolically and environmentally by osmosis. We can get uncomfortable or sick if we drink polluted water or if the air we breathe does not fit our preferences. The general necessities of humans may be too still, windy, hot, cold, humid or dry for our liking, not have enough oxygen, or be polluted with toxic gases or smoke. However, air is fairly uniform in composition throughout the world. Although you may find the climate unpleasant, you can survive as well in one part of the planet as in any other.
However, water is extremely variable in its makeup. It can be hard or soft, acid or alkaline, fresh, brackish, or saline, pure or polluted, rich or poor in oxygen. Unlike humans, fish cannot survive in just any environment. The systems of fish are specialized, and unable to deal with any change. They may have discomfort and poor health, or even die if the water they drink, breathe and live in does not meet their specific necessities, primarily if the change is sudden.
Therefore, we must never believe that water is a uniform chemical, or that all fish will survive happily in the water from the tap. The owner of fish must be a water technician giving the right environment to the fish in terms of temperature, chemistry, movement, oxygen content and freedom from pollution. There are many tools and test kits for keeping track of the quality of aquarium water. There is also a large variety of tools for adjusting the quality, chemistry and other factors so that there is no reason for trial and error water management.
Water Chemistry in your Fish Tank
This term refers to the combined hardness, salinity, and acidity of water. For fish to live in water its metabolism has to be in tune with these aspects of its environment. Successful adaptation to major changes typically requires gradual alterations, allowing time for metabolic adjustment. While some fish can be adapted to water chemistry not natural for them, this might prove to be dangerous to the fish in the long term. For example, the difference between salt water and fresh water is too large to overcome.
There are two reasonable ways to approach the question of water chemistry. First, you can keep fish suited to the water you can provide them from the tap. Second, you can modify your local water to make it adaptable for more particular species. Modifying the chemistry of water can be somewhat costly and time consuming. There is also the possibility of mistakes, so it is suggested that new aquarists take the former lessons. It is not recommended that you try to modify the fish.
Sudden changes in hardness or PH are the most likely to damage fish. Even natural water chemistry can be dangerous if the fish’s system has adjusted to other environments. Local pet shops typically use tap water, so it is best to do this initially and gradually adjust the chemistry of the water after introducing the fish to their new environment. Later on you can buy additional fish, which may be accustomed to different conditions than what you have in your aquarium. In this case you should set up a temporary aquarium with an environment suited to the new fish, and you can gradually adjust the chemistry of the water to match your main tank.
You may also be told that minimal chemical shock will occur if you mix small amounts of your tank water with the water in the bag prior to letting the fish out in the tank. This is not true, because the required metabolic adjustment takes days or weeks, not just minutes.