Old Wives Tales

Tap water is safe for fish if you leave it out overnight – NOT!  While this used to be true in some areas of the country, it has long since become a thing of tails.  Years ago most municipalities used chlorine to sterilize their drinking water.  Chlorine forms a weak bond with water and will dissipate in about 24 hours, especially if the water is moving.  That’s why our drinking water is now treated with chloramines; chlorine bonded with ammonia.  Chloramine will stay in water for 34 days, keeping our supply of drinking water safe for much longer and at a lower cost.  Like chlorine, chloramines are highly toxic to fish, corals and inverts.  The only way to ensure tap water is safe for fish is to use a commercial dechlorinator with a sodium thiosulfate base.
Note:  Some dechlorinators can interrupt the effectiveness of your protein skimmer.  Seachem Prime does not effect protein skimmers at all, so is the best choice if you have a saltwater tank.  Also, during spring run off, the water treatment plants put more treatments in the water, so it is recommended to use a stronger dechlorinator dose when dechlorinating the tap water.