Soda water is good. Vinegar can work but may lower O2 and cause a bacterial bloom.
Here’s some info:
https://www.reefedition.com/ph-and-the-reef-aquarium/
Solutions to High pH Problems
Some solutions to pH problems are peculiar to a specific cause, such as adding vinegar directly to limewater, or using less limewater than normal. Some general solutions, however, are frequently effective. Water changes are generally not an effective long-term solution to any pH problems. My recommendations on how to deal with high pH problems are detailed below.
Adding a buffer is a very poor way to control high pH. The best option in this regard is to add straight baking soda, but it lowers pH only slightly and provides a large boost to alkalinity. I showed experimentally in a previous
article that adding enough baking soda to lower artificial seawater’s pH by 0.04 pH units raises alkalinity by 1.4 dKH (0.5 meq/L).
The most benign way to reduce high pH is to aerate the water more. Whether the aquarium looks well-aerated or not, and regardless of its oxygen level, if its pH is above 8.5 and its alkalinity is below 11 dKH (4 meq/L), then the aquarium is not fully equilibrated with carbon dioxide in the air (if its alkalinity is much higher than 11 dKH, then that may also require correction). Equilibrating carbon dioxide can be much more difficult than equilibrating oxygen. Air contains very little carbon dioxide (about 350 ppm) relative to oxygen (210,000 ppm). Consequently, a lot more air needs to be driven through the water to introduce the same amount of carbon dioxide as oxygen. Perfect aeration will solve nearly any high pH problem, and will rarely cause any problem of its own.
That said, sufficient aeration is not always easily accomplished, and other methods can be useful. These other methods are:
- Direct addition of carbon dioxide: Bottled soda water (seltzer) can be used to instantly reduce an aquarium’s pH. Be sure to select unflavored soda water, and check its ingredients to be sure it doesn’t contain anything that should be avoided (phosphate, etc.). Many manufacturers list water and carbon dioxide as the only ingredients.
I recommend adding 6 mL of soda water per gallon of tank water to reduce pH by about 0.3 units. Add it to a high flow area away from organisms (such as in a sump). The local pH where it first is added will be very low. Going about this procedure slowly is better than proceeding too fast. If you do not have a sump, add it especially slowly. Some soda water may have more or less carbon dioxide in it than others, and the lower the aquarium’s alkalinity, the more the pH will drop. Also, the higher the pH, the less the pH will drop, because seawater’s buffering capability declines steadily as the pH drops from about 9 to 7.5.
Figure 3. Adirondack Seltzer, used to add carbon dioxide and lower pH.
- Direct addition of vinegar: Commercial distilled white vinegar (typically 5% acetic acid or “5% acidity”) can be used to instantly reduce an aquarium’s pH. Do not use wine vinegars because they may contain undesirable organics in addition to the acetic acid.
I recommend adding 1 mL of distilled white vinegar per gallon of tank water to initially reduce pH by about 0.3 units. Once again, add it to a high flow area away from organisms (such as in a sump). The local pH where it first is added will be very low. Going about this procedure slowly is better than proceeding too fast. If you do not have a sump, add it especially slowly. The lower the aquarium’s alkalinity, the more the pH will drop. Also, the higher the pH, the less the pH will drop, because seawater’s buffering capability declines steadily as the pH drops from about 9 to 7.5. Remember, there may be an additional later drop in pH as the aquarium’s organisms metabolize the vinegar to carbon dioxide.
Figure 4. Heinz Distilled White Vinegar, used to lower pH.
- Addition of vinegar via limewater: Commercial distilled white vinegar can be used to reduce a tank’s pH by adding it to limewater that is subsequently added to the aquarium (instead of using limewater alone). Do not use wine vinegars because they may contain undesirable organics in addition to the acetic acid. A reasonable dose to start with is 45 ml of vinegar per gallon of limewater.
