Vinegar And Limewater To Reduce pH
The reason that limewater raises the pH of aquarium water so considerably is because of the hydroxide that it adds. As described above, the hydroxide can combine with carbon dioxide to form bicarbonate and bring the pH back down. In many aquaria, however, the aeration is not great enough to bring in carbon dioxide fast enough to meet all this demand, and the pH rises. There are several ways to add additional carbon dioxide to meet this demand, including delivery from a carbon dioxide cylinder. Many aquarists, however, choose to add carbon dioxide in the form of vinegar. Many of them choose to add the vinegar directly to the limewater, although if pH reduction is the goal, it can also be added directly to a high flow area of the aquarium.
When vinegar is added directly to aquarium water, the active ingredient is acetic acid. The first thing it does is ionize into acetate and H+:
6. CH3COOH àCH3COO- + H+
Bacteria can then metabolize the acetate to gain energy in the reaction shown below:
7. CH3COO- + 2O2 à2CO2 + H2O + OH-
On balance, the H+ released in (6) and the OH- released in (7) offset each other, and the net addition is simply carbon dioxide:
8. H+ + OH- àH2O
9. CH3COOH + 2O2 à2CO2 + 2 H2O
One of the potential side effects of this metabolism is that the bacteria performing the transformation may grow faster because of it. This growth may have positive or negative outcomes. One potentially positive outcome is that as they grow, they will necessarily consume nitrogen and phosphorus, possibly lowering nitrate and phosphate levels in the aquarium. Another is that the bacteria may be a suitable food source for other organisms.
Potential drawbacks can include reduced oxygen as the bacteria use it to consume the acetate, and the appearance of unattractive bacterial mats in the aquarium (reported by some, but not by the majority of vinegar users).
