Dechlorinator (water conditioner) is a liquid additive that instantly neutralises chlorine and chloramine in tap water, making it safe for fish and filter bacteria.
A few drops per bucket is all it takes, and it works immediately. Some conditioners also temporarily detoxify small amounts of ammonia, which can help during a fish-in cycle.
It is the one product every keeper needs on hand for every water change — running out is not a reason to skip treating the water.