pH Shock

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pH shock is a condition that can kill or seriously harm any aquatic animal. It is brought on by a rapid change of the pH of the tank water within too short a time. This usually happens because of several conditions.


  • Very Large water changes performed when the old water has a large pH difference to the new water.
  • CO2 excess during the night or when the supply of CO2 changes and the KH level in the water is too low (typically less than 4dKH (~70ppm)) so causing large pH changes. Usually this happens when your water is soft.
  • Chemicals added which alter the pH too much.
  • Transporting a fish in a plastic bag for too long (say greater than 3 hours), and then opening the bag, can cause a rapid pH shift within 20 minutes as gases trapped in the water are released. This can seriously shock or kill the fish if left in the bag.
Adding a measure of water conditioner before transport can greatly reduce this effect. Or simply do not open the bag until ready to release the animal.

The amount of a sudden change of pH is tolerated differently by different species of aquatic animals, and one should try not to find the limits for one's pets.


Fish will become lethargic, they stop eating, their dorsal fin drops and they remain relatively motionless or rest on the substrate. Some may produce excess slime on their bodies giving a off colour or off-white look to their bodies. More easily seen if the fish is dark in colour.

Within a few days if the pH shock continues then the fish will usually die.


Measure the KH and pH levels with a good test kit, strip based kits are often inaccurate at measuring KH at low KH levels. If the KH is less than 4d then add small amounts of potassium bicarbonate or sodium bicarbonate every hour until the KH level is higher and the pH has not changed by more than 0.3.

  • Do not add pH 'up' or 'down' adjusting chemicals from a bottle in order to try to return the pH to a known value as this is meaningless. The idea is to buffer the water so that the pH remains stable at whatever value is natural for your tank environment and local water.

If the KH level is raised, then any pH shift at night will be minimised and the fish will most likely recover if it's not been too long. Note however that a pH shocked animal has a low immune system and may be susceptible to other diseases. It may take it several weeks to fully recover.

  • If KH is high and the pH is still changing rapidly over a few hours then you will need to investigate the cause of this before your animals suffer. This can be caused by chemicals leaking out of ornaments, possibly something leaking into the tank water like gases from the substrate.