Why does this happen and how do I prevent it?
Water hardness is a measure of the total dissolved salt content in water or the quantity of certain metallic ions. E.g. calcium and magnesium. Water hardness and pH have a very important relationship in the pond. Fish require a stable pH to thrive and this is easier to provide when the KH (water hardness) level is high. Hydrogen ions cause the pH to drop. These are released as fish breathe and digest food. A high KH acts as a sponge to soak up these ions and prevent the pH from dropping.
Incorrect water hardness has a significant impact on fish and can affect their ability to maintain an internal balance of salt and water, their blood pH, kidney function and egg development.
If your tap water has a high KH then water changes should be used to prevent the KH from dropping too low. If your tap water has a low KH you can add a pH buffering substrate such as crushed cockle shells.
What treatment should I use?