You’ll hear a lot of different philosophies on changing water in an established pond. Some hold that once a system is in balance, you shouldn’t need to do any water changes at all. That’s the “if it ain’t broke, don’t fix it” school of thought. Others believe in doing massive water changes on a regular basis to control for the accumulation of fish waste, ammonia, nitrates, etc. I guess that’s the “better safe than sorry” model. As in most things in life, moderation is probably the best policy. Frequent massive water changes can stress your fish, disturb the colonies of beneficial bacteria, and even lead to general maintenance fatigue on your part! On the other hand, a strict policy of “don’t touch” doesn’t take into consideration the potential accumulation of pollutants in your pond. As your water evaporates slowly over time, it can be replaced with fresh, treated water, but any pollutants (think smog particles) that your filter can’t handle will gradually become more concentrated and could eventually lead to real problems.
The middle ground probably means regular small water changes - about 10% of your total water volume, perhaps weekly or monthly, depending on how your pond is doing and how enthusiastic you are. The best method is to draw the appropriate amount of water ahead of time - you can store it in a plastic kiddie pool, for example, which gives it a chance to equalize with the outdoor temperature. Go ahead and treat the water with a good water conditioner to deal with chlorine and chloramine, heavy metals or other potential hazards; test and correct for any chemical outliers like salt or pH; and pop in an aerator to optimize off-gassing. These preparations ensure minimal stress for your fish. Keep in mind, as well, that you can use the pond water you just removed for a lovely burst of nutrients in your garden!
