Tap water contains chlorine and chloramines which are toxic to fish and beneficial bacteria. Dechlorinators (or water conditioners) are essential for every water change.
Modern conditioners use chemical reduction to neutralize chlorine and break the bond of chloramines, rendering the ammonia component safe until it can be processed by your bio-filter.
If you're new to the hobby, one of the first things you'll hear is: "Never put fish in straight tap water." But why? To us, tap water is clean, safe, and life-sustaining. To a fish, however, untreated tap water is essentially a chemical burn waiting to happen. Understanding how dechlorinators work isn't just about following rules—it's about ensuring the literal foundation of your fish's world is safe.
To keep our drinking water safe, municipalities add disinfectants like Chlorine and Chloramine. These chemicals are designed to kill bacteria and pathogens so we don't get sick. Unfortunately, they are incredibly effective at killing the beneficial bacteria in your aquarium filter, too. More importantly, they attack the sensitive gills of your fish, leading to stress, suffocation, and eventually death.
While chlorine eventually evaporates (if you leave water out for 24-48 hours), Chloramine is much more stable. It’s a bond of chlorine and ammonia that never evaporates. Using a modern water conditioner is the only reliable way to break that bond and keep your tank safe.
There are two main ways to dose dechlorinator, and getting the math right is where most people stumble:
While most modern conditioners like Seachem Prime are incredibly safe, even up to 5x the standard dose in emergencies, they are still chemicals. The primary danger of massive overdosing is oxygen depletion. Water conditioners are "reducing agents" that consume oxygen as they work. In a tank with poor surface agitation, a massive overdose could potentially suffocate your fish.
On the flip side, under-dosing is the far more common danger. If your city suddenly "flushes" the lines with extra chlorine, a standard dose might not be enough. This is why many pros dose a tiny bit extra just for peace of mind.
Ever notice your dechlorinator smells like rotten eggs? That's actually normal for many sulfur-based conditioners! However, they do have a shelf life. Most liquid conditioners are good for 2-3 years. If you notice a color change or strange clumps, it’s time to grab a fresh bottle. Your fish’s lives are worth more than the $10 for a new bottle of Prime.
If you add water to the tank first, you must dose for the total volume of the tank. If you dose the bucket before adding, you only need to dose for the bucket volume.
Some cheap conditioners only remove chlorine. In cities with chloramines, this leaves toxic ammonia behind. Always use a "Complete" conditioner.
Use our precision calculator to find the exact dose for your tank's net volume.
Go to Dosage Calculator
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