Summarize the following:
Run some water from your sink into the bucket, leaving 2–3 inches (5.1–7.6 cm) of space at the top to make the bucket easier to carry without sloshing. It’s best to use warm or lukewarm water, as it will be closer in temperature to the water that was already in the tank previously.  Use whatever water source is closest to your aquarium to cut down on the distance you’ll have to tote the bucket back and forth. You may need to fill the bucket multiple times if you’re replacing the water in a particularly large tank. Refer to bottle of the product you choose for more detailed directions. Most of the time, it will only take 1-2 drops per 1 gallon (3.8 L) of water to successfully chlorine, chloramine, ammonia, nitrite, and other chemicals and impurities. If left untreated, the chemicals in tap water could wipe out whole colonies of good bacteria, or even harm your goldfish itself. Carefully lift the bucket over the edge of the tank and tilt it so that it flows in little by little. When refilling a large tank, wait 5-10 minutes before adding every next bucket to give the water time to reach a consistent temperature. Continue until you’ve replenished the water to its original level.  If you’ve got a thermometer in your tank, look for it to reach a minimum of 60 °F (16 °C) before adding more water. Goldfish are most comfortable at around 65–72 °F (18–22 °C). Unless the water is near room temperature, it may be too cold or too warm for your goldfish.  Raising or lowering the temperature in the tank drastically could "shock" your goldfish, leaving it vulnerable to disease. Now that your aquarium is clean, all that’s left to do is reactivate the filter, heater, and air pump. If possible, let everything run for about 20 minutes to bring the water temperature and oxygen back to their optimal levels. Afterwards, return your goldfish to the tank and welcome it home with a snack! Remember to plug your goldfish tank back into the nearest wall outlet.

Summary:
Fill your second, clean bucket with tap water. Add a dechlorinator to the tap water to make it safe for your goldfish. Pour the treated water into the tank gradually. Turn your tank equipment back on.