Chlorine at high concentrations is toxic to fish; at lower concentrations, it stresses fish by damaging their gills. Concentrations of as little as .2-.3 ppm kill most fish fairly rapidly. To prevent stress, concentrations as low as 0.003 ppm may be required. Fortunately, chlorine can easily be removed from water by the chemical sodium thiosulfate, readily available at fish stores under various brands. Sodium thiosulfate neutralizes chlorine instantly. Note that there are many “water treatment” products that are advertised as “making tap water safe”. Read labels carefully. Inevitably, the ones that neutralize chlorine all contain sodium thiosulfate, plus other substances that may or may not be useful. If your water only contains chlorine (as opposed to chloramine), sodium thiosulfate is all you need.

The most cost-effective treatments use only 1 drop per gallon of water. Most other water treatments are much more expensive in the long-term; they may require a teaspoon of treatment (or more) per gallon!

Chlorine is relatively unstable in water, escaping to the atmosphere on its own. Water left in a bucket (or tank) with adequate water circulation (e.g. filter or airstone) will be free of chlorine in 24 hours or less.

Many netters report that they perform partial water changes without ever treating their tap water to remove chlorine. Keep in mind that even though fish show no APPARENT ill-effect from untreated water, that doesn’t mean that the chlorine isn’t stressing your fish. How much stress depends on how much chlorine is introduced to the tank, which depends on many factors (including the percentage of new water added). Because chlorine removers are so cheap (pennies per usage), the insurance they provide should not be passed up.