how about adding calcium hydroxide (limewater, kalkwasser) to help with lower PH? Never done it in a very large tank, but could help.
Thoughts?
Low pH Due to High Indoor Carbon Dioxide Levels
High indoor carbon dioxide levels can also 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 the wind can be felt blowing around old window frames.
Many aquarists have found that opening a window near the tank can significantly raise the pH within a day or two. Unfortunately, those aquarists living in colder climates cannot comfortably 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. Be advised, though, that if the aquarist happens to live in an area where insecticides are periodically sprayed for mosquito control (such as in many metropolitan areas of the South), it is important to place some type of carbon filter at the air intake to prevent these chemicals from entering the aquarium.
Finally, the use of limewater in these situations can be an appropriate solution. Limewater may be especially effective in this situation because the tank would be less likely to experience the undesirably high pH that sometimes accompanies limewater use. While limewater is a common aquarium 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 (Bicarbonate B-ionic). For
home brews, washing soda (sodium carbonate) or baked baking soda would be better than normal baking soda (sodium bicarbonate).
