How to Recognize, Correct a pH Crash in Your Koi or Goldfish Pond
pH crashes occur when the water in the pond is not stable (also called alkalinity) and the pH suddenly plummets down below 7.0. We have seen it as low as 5.0. Your pond's pH is at its lowest level during the morning so a pH crash most often happens around or just before daybreak. Small bodies of water are less stable than large ones so a crash is more apt to happen to smaller ponds (of less than 2000 gallons), holding, hospital or quarantine tanks and aquariums.
Low pH is acidic. A sudden drop would feel to the fish like they were being lowered into a vat of acid. If pH was temperature - it would be hot. A pH crash kills fish, damages plant life and kills the nitrifying bacteria in your biological filter.
Recognizing a pH Crash
The tell-tell signs of a pH crash are: Skin peeling on the fish / Finding all the fish dead in the morning / A reading of less than 6.8 when tested after the episode.
Correcting the pH
pH changes are harmful and often we will say to gradually change the pH of your pond so that it doesn't shock the fish. In the case of a crash you have to think about the "fire" that any surviving koi or goldfish are feeling and it's the lesser of two evils to get that pH up and fast! If there are no survivors then you can certainly change it out at your convenience but if there are surviving koi or goldfish you need to treat it as an emergency and act quickly. When changing the water remember to add de-chlorinator if you are using a municipal water source.
If you know you've had a pH crash and there are surviving koi or goldfish do an immediate water change to bring the pH to a normal range. (Don't forget the de-chlorinator if necessary)
If a water change cannot be done immediately, add baking soda to bring up the pH every 30 minutes until 7.0 is reached or use pH Up.
What to Expect Next
Once the pH is corrected after a crash expect problems with ammonia and nitrites. A pH crash kills the nitrifying cycle of your pond and you must start re-seeding your biological filter immediately. The surviving koi and goldfish of a pH crash will be stressed to their limits and they are still not out of danger. Any underlying problems (scarred gills, presence of parasites, bad water quality, etc.) will affect the fish much more than if the fish were not stressed. Try to keep the stress factors down and watch the fish carefully for signs of secondary problems.