Please, please keep a bottle of dechlor on hand. If you never need it, that's wonderful, but here's why you need it. You turn the water on to top off your pond. It's going to take a bit of time, so you decide to fold the laundry while you are waiting. The phone rings. You chat with your friend for a few minutes. Then you remember you have to get some bill payments in the mail, so you hop in the car to drop them off at the post office. While you are out, you decide to pick up a few things at the grocery store and pick up the dry cleaning. In the cleaners, you talk with the clerk about the weather for a few minutes, get back in your car and see you need gas, so you stop to fill up the tank. You get home and for the life of you, you can't figure out why the driveway is flooded.
Suddenly it dawns on you. You rush to turn off the water and see your fish lying at the bottom of the pond not moving. If you have dechlor in the house, you can probably save those fish.
Whew, aren't you glad you have some?