The amount of water conditioner needed for a water change depends on the volume of water being changed. Generally, you should dose the water conditioner based on the new volume of water being added to the aquarium, not the total volume of the tank. For example, if you are performing a 25% water change on a 10-gallon tank, you will be replacing 2.5 gallons of water. You would then add the appropriate amount of water conditioner for 2.5 gallons of water, based on the dosage instructions provided by the manufacturer.