In the natural environment, tropical fish often live in a massive volume of water such as a river or lake.
If the sun shines, it will take days to alter the average temperature. Fish and plant waste may decompose producing ammonia and nitrogen. if it is so heavily diluted it couldn’t possibly hurt the fish. Even the worst disaster is very heavily diluted.
In the aquarium the opposite is true. The tiniest change affects a small volume of water really quickly. This means we need to care for our water if we want our fish to survive.
The first thing to consider is the tapwater you’ll add when you first fill the tank, and in subsequent water changes. Tap water contains chlorine in many areas and chloramine in others. Chlorine is a gas that will naturally escape if water is left to stand. When you first complete a tank water fill, just leave the water alone for a couple of days and the chlorine with have dispersed.
When doing water changes, either leave the water to stand or add a neutralizing liquid (For reasons of volume, it’s better to use a neutralizing liquid). In some water areas, the tap water is sterilised using chloramine. This is harmful to fish, plants, and friendly bacteria.
Cholamine is fixed into the water supply and will not be removed by leaving water to stand. A tap water neutralizing liquid has to be added to tap water containing chloramine. To find out more about your tap water visit your local water company’s website. Most have an interactive feature where you input your postcode to see the latest chemical analysis of your tap water.
Resources: Fish tank Setups