What is the Best Aquarium Water Dechlorinator?
Chlorine in Tapwater
Chlorine is added to tap water at a concentration of 0.2–1 mg/litre. Chlorine kills bacteria and other dangerous microbes and prevent the spread of disease. While some people consider chlorinated water is bad for human health, it is generally considered safe to drink at these concentrations. However this concentration is toxic to most fish and other aquatic organisms. Adding livestock directly to tap water-filled aquariums will kill most commonly kept aquarium species.
Alternatives to Dechlorinating Chemicals
You may find some people report adding tap water to their tanks with no ill effects. Often they are just topping their tanks off. This means the chlorinated water is being diluted by the chlorine-free water already in the tank. This may reduce the concentrations below the level toxic to fish but it is a risky practice.
Because chlorine is volatile, it can also be removed by leaving water in a bucket overnight before adding it to the tank. However in some areas chloramine is used and this is less volatile and so gassing off may not be an effective method.
Many companies make an dechlorinator and any of them will be adequately effective. This includes API Tap Water Conditioner, Tetra Aquasafe Water Conditioner, and Aqueon Tap Water Conditioner. Avoid brands that are not well-established and not aquarium product specialists. Beware of counterfeit products especially when buying online. It is best to buy directly from the manufacturer or an official supplier of their product.
I would recommend avoiding multi-purpose products that also contain products meant to add live bacteria, improve fish health, or treat disease symptoms, as it is difficult to know how these ingredients will effect various different kinds of aquatic life.
After trying a few products I now exclusively use Seachem Prime for the following reasons:
1) It is safe and effective. I have used it for over twenty years with fish, invertebrates, and snails will no ill-effects.
2) It can also be used to remove ammonia if you have an unexpected ammonia spike in your tank. It is also effective against chloramine which is used in some areas rather than chlorine.
3) Given that it is the most highly concentrated dechlorinator the price is reasonable. If your budget is limited you can buy a smaller 100ml bottle for less than $5. A more concentrated product is also cheaper to ship, and easier to store. You need only 1ml to treat 10 gallons of water.
4) It is very difficult to overdose.
5) It is made by a reputable company that is unlikely to have any quality control issues.
6) It is very easily and widely available.
If you wish to double-check my recommendation check in at any reputable aquarium forum, facebook page or local club and ask the people there which dechlorinator they use. You will typically find that 80% or more prefer Prime. The only downside you might see mentioned is a slight "rotten eggs" smell. I personally find this product to have only a faint odor in concentrated form and none once added to the tank.