A dechlorinator, also known as a water conditioner, is a chemical solution used in aquariums to remove chlorine, chloramine, and other harmful substances from tap water. Tap water often contains chlorine or chloramine added by municipal water systems to kill bacteria and other pathogens. However, these chemicals can be harmful to fish, invertebrates, and beneficial bacteria in an aquarium.

When added to aquarium water, a dechlorinator neutralizes chlorine and breaks the bond between chlorine and ammonia in chloramine, rendering these substances harmless. This process makes the water safe for fish and other aquatic life.

Dechlorinators often also detoxify heavy metals and may sometimes contain additional ingredients to reduce stress in fish, promote slime coat health, or enhance water quality. Using a dechlorinator is essential whenever setting up a new aquarium, performing water changes, or adding new water to an existing tank.