11-23-2012, 11:56 AM
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I agree with beaslbob that the fluctuation is probably the normal diurnal cycle, depending when you are testing, and this is not a problem. It occurs in most natural waters and fish are used to it. Always measure the pH the same time each day in order to get a clearer idea of any fluctuations aside from the afore-mentioned. And try to do it in daylight as the colours will be more reliable; all artificial light sources can distort them.
The article Geomancer linked is my summation of the issue [and Reefing's linked article is one of the sources I used] and as it notes therein, diluting the water is the safest method. Any additives will carry risks, depending.
Each tank is biologically unique however. I have 7 tanks running, all receiving 50% water changes weekly with tap water that is near-zero KH and GH and a pH of 7. Plant fertilization including a GH booster (Equilibrium) is identical in all tanks. Yet some remain at pH 6.4-6.6, one or two at 6.8, and one at 7.2. And these are very stable, according to periodic tests carried out prior to the water change in early morning, and over a period of months.