Dechlorination
Chlorine is toxic to all fish, causing severe gill damage, leading to asphyxiation and death. There is no effective treatment for chlorine poisoning in fish, so exposure is likely a death sentence. Therefore, it is important to always treat your water.
IMPORTANT! This is not applicable if your water is treated with chloramine, in which case a filtration system is the only option.
Dechlorinator
Before adding fresh water from the tap into the tank, dechlorinate it using a store bought dechlorinator, 5-10 minutes prior to moving it to the tank. For larger tanks that require a hose pump to refill the tank water, do add the dechlorinator at intervals to prevent overdose.
Home Solution
In the event that there is no dechlorinator available, and the water from your tap is treated with chlorine, leave the water to air for at least 24 hours before adding the water to the tank. This minimises the concentration of chlorine as it will “bubble off” since it is volatile, and make it safe for fish.
While this solution is still tenable, a dechlorinator is always preferred.