Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Why is it fluctuating?
Are you adding CO2?
Is your tap water variable?
Do not add much (if any) calcium carbonate (coral sand, oyster shell grit, limestone) or other source of carbonates. You might have to add a small amount. Try to maintain a KH of about 3 German degrees of hardness, perhaps 2 degrees.
Carbonates are a buffer that will help to stabilize the pH. Too much carbonates, and the pH will be buffered at a higher level.
Organic matter such as peat moss, oak leaves, Indian Almond leaves (most leaves, in fact) and some types of driftwood can lower the pH.
Some peat moss can act like a cation exchange water softener, and will remove certain positively charged minerals and add H+ in its place. Rising H+, of course, means lower pH unless there is something else in the tank that removes the H+. I keep some Canadian Sphagnum Peat Moss (garden sort of stuff) in the filters of my soft water tanks.
If these measures do not help, then we return to the original questions.
Why is it fluctuating? If you can minimize whatever is going on, then you minimize the fluctuations, hopefully before adding the water to the tank.
Are you adding CO2? If the fluctuation is because of the daily cycle of CO2, do not worry about it. If the mineral levels in the water are appropriate for the livestock (TDS, GH and KH), then variable pH because of CO2 is not a problem at all. (Only in the fishkeeper's mind. Fish are OK with it.)
Is your tap water variable? If so, then you may need to prep the water ahead of time. Treatment needed will depend on what is going on with the water. If you are on a municipal water source, talk to the company and find out what is going on. If you are on a private well, you might have to do some testing and figure it out.