When you’re trying to protect your fish while cleaning your pond, you should keep vinegar out of the water unless you’re adding it in very small amounts. Vinegar is commonly recommended as an additive to change the pH or kill off unwanted bacteria that create unpleasant odors. While plain 5% household vinegar can be used for cleaning a pond after it has been drained, it shouldn’t be added as a water purification chemical. It’s an acidic liquid but it’s not effective at changing the pH without a large volume being added to the water.
After adding enough vinegar to the water to see a pH change, you’ll also notice your fish are having trouble getting enough oxygen and their gills may turn red and irritated. Acetic acid is irritating to sensitive fish like koi and goldfish, especially when added in any sort of volume. Definitely do not add any vinegar attempting to control odor issues. Odors are caused by bacteria and high waste levels. Vinegar breaks down into organic material much like fish waste, further feeding bacteria and causing more odor. Apple cider vinegar is even more likely than other types to cause this problem and no safer for your fish.
