pond water chemicals

Question
We were topping our Koi pond up with water from the hose pipe and got distracted. It was therefore left on overnight and the pond chemistry is now out of balance. Although we are going to put more chemicals in today my husband thinks that the damage has been done to the fish and that they will now all die. Is this true and is there anything we can do to aviod this.

Answer
Unless the water is from a well, it will usually contain chlorine.  Chlorinated water is toxic to fish unless you add dechlorinator.  Chlorine damage is fairly irreversible.  The best thing you can do for the surviving fish is to make sure the pH stays stable and that there is no ammonia.  This will help keep the fish free of bacterial infections.  You should also add some salt to the pond.  You want it to be about 0.20 ppt.