ph and alkalinity imbalance

[ai[k²]wan]

decided to ask this on a separate thread.

tanzy mentioned too much vinegar in saturated kalk would cause this, but how?

[Tanzy]

When you use kalkwasser, you are adding calcium and hydroxide ions. Hopefully, if all things turn out well, the hydroxide will react with dissolved carbon dioxide to give you carbonates, which is the alkalinity part of the equation.

Adding kalkwasser with vinegar (acetic acid), decreases the hydroxide ions and increases acetate ions. Usually, the whole solution should be just calcium acetate and water. Acetate can also act as a buffer like carbonate, but unlike carbonate which buffers the pH at around 7-8, acetate buffers water at around 4. Acetate is broken down by bacteria to carbonate but if you add too much, the bacteria can't break it down in time resulting in excess acetate in the tank. As some overzealous vinegar users have discovered in the past that the tank pH is always depressed.