Tap water straight from the hose is dangerous to fish and to the bacteria a pond depends on, because of the chlorine or chloramine added to make it safe to drink.
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The Fix
A dechlorinator (sold as a liquid conditioner) neutralizes chlorine and chloramine within minutes. Letting water sit out only removes chlorine, not chloramine, which many municipalities now use instead.
The Common Mistake
Assuming untreated tap water is fine because ‘it’s just a small top-up’ is how healthy biofilter colonies get wiped out during routine maintenance.
