Partial water change outs can reduce the amount of anything dissolved in the water but not totally remove it. Although it is sometimes necessary, draining and refilling a pond should only be used as a last resort! Do not use large water change outs to clear green water conditions. A large water change out will normally make the situation worse, not better. Often, several partial water change outs, performed over a period of days or even weeks, can reduce the concentration of an undesired item to acceptable levels without serious after effects. A water change out reduces the amount of a substance in the water by the same amount as the percentage of water replaced. Remember the concentrations of any "good" stuff in the pond is being reduced at the same time as the "bad" stuff. Also the water being used for replacement may have undesirable components as well.
A water change out is considered to be when a measured amount of water is removed from the pond and then replaced. Just adding water and letting the pond overflow will not accomplish the desired results unless significantly more water is transferred. Water added to replace that lost by evaporation is not part of a change out.
Example: It is desired to decrease the Salt in a pond by one half. Any of the following will have the same approximate result:
Seven successive 10% change outs.
One 25% change out followed by four 10%.
Two successive 25% change outs followed by one 10%.
One 50% change out.
Depending on the urgency to carry out the action, the largest number of change outs over the longest time would be the best approach.
Unfortunately, this does not apply in the same way to pH. The change in pH for a given water change is dependent upon the Alkalinity and pH in the pond as well as the Alkalinity and pH of the replacement water. Adding water with a higher pH than the pond water will raise the pond water pH but it is difficult to predict how much. Remember that if the water being added to the pond has a pH higher than that of the pond, make sure any ammonia in the pond has been treated before adding the new water.
It is considered appropriate to change out from 5 to 10 percent of a pond's water per week. A small pond (500 gallons or less) should receive the 10% weekly change out. The 5% change out is appropriate for larger ponds (5000 gallons or more). Any water replaced after a back flush of a filter system or other maintenance actions can be included as being part of this weekly change out amount.
It is very common for pond keepers to skip making these routine water changeouts. After all the pond is full of water. Why should we just dump some of it and refill it? Many components in the water build up over time and this is the only way to reduce them. Experienced pond keepers know that their fish are healthier and stronger when these water changeouts are conducted.
When making the water replacement with tap water that contains chlorine and/or chloramine, it would be better to pretreat the water with the chlorine neutralizer before adding it to the pond (particularly small ponds). If this cannot easily be done, use a fine spray of water over the pond and divide the total computed neutralizer dosage into two to four parts and add while the makeup water is being added. Don't Forget To Turn Off The Water! Set a timer or do something to remind you that makeup water is running into the pond. Inexpensive flow timer shutoff devices that hook directly on the hose are available and are a good safety item to use.