API Tap Water Conditioner

"API TAP WATER CONDITONER treatment removes toxins from tap water instantly, 
so you can add fish to your aquarium (or add them back after a water change) 
immediately. It dechlorinates aquarium and tap water, detoxifies heavy metals, and 
may be used in both fresh and salt water."

Recommendations

"This super-concentrated formula treats up to 600 US gallons with just one 
 ounce, making your aquarium a safe environment for your fish. Add 1 ml. 
 for each 20 gallons of aquarium water."