I agree with "I recommend adding 1 mL of distilled white vinegar per gallon of tank water to initially reduce pH by about 0.3 units ", but that will be very temporary if dripping kalk is the cause of your PH problem. I had to add around 125mL of vinegar to 2tsp kalk dissolved in 1 gal H20 to reach an acceptable PH range so my kalk drip wouldn't cause a ph rise in my already top of ph range tank. Adding 45mL still had my kalk solution with a ph well above 12.