Other reasons for low pH: by Randy H Farley
Low pH Due to High Indoor Carbon Dioxide
High indoor carbon dioxide can also apparently lead to low pH problems in many tanks. Respiration by people and pets, the use of un-vented appliances burning natural gas (e.g., ovens and stoves) and the use of CaCO3/CO2 reactors can lead to high indoor carbon dioxide levels. The level of carbon dioxide can easily be more than twice that of exterior air, and this excess can substantially lower the pH. This problem is especially severe in newer, more airtight homes. It is unlikely to be a problem in homes like mine where you can feel the wind blowing around old window frames.
Many aquarists have found that opening a window near the tank can result in a significant rise in pH within a day or two. Unfortunately, those aquarists living in colder climates cannot readily open windows in the winter. Some have found it useful in these situations to run a pipe or tubing from the outside to the air input of a skimmer, where fresh, exterior air is rapidly mixed with the tank water.
Finally, the use of limewater in these situations is also appropriate. In fact, limewater may be more optimally used in this situation because the tank would be less likely to experience the undesirably high pH that sometimes accompanies limewater use. While limewater is the alkalinity supplement most potent at raising pH, other high pH additives would also suffice. Supplements based on carbonate, for example, would be very useful in this situation, while bicarbonate would not be. As a commercial example, the original B-ionic would be better than the newer version. For home brews, washing soda (sodium carbonate) would be better than baking soda (sodium bicarbonate).
Low pH Due to Low Alkalinity
Low alkalinity can also lead to low pH. For example, if alkalinity is not supplemented as fast as it is removed by calcification, the pH will likely drop. This drop will happen with all alkalinity supplementation schemes, but will be most observable using schemes that do not themselves raise pH (like CaCO3/CO2 reactors or bicarbonate). In this situation, the obvious solution is to add more alkalinity in some fashion.
