I have it in one of my kapampa tanks, but since I don't rely on the sand alone to buffer ph (I add baking soda with water changes) I don't know a definite answer to that. I suspect: 1) from the substrate alone it's not immediate, 2) it depends on the overall buffering chemistry in your tank, 3) it depends on the volume of water you're exchanging and the properties of the new water. In other words imo you'd have to do trial and error with a specific tank to see what it does in
that tank.
Not directly related to your question but of some relevance is that fish are not as sensitive to ph changes,
within their range of tolerance, as some would lead you to believe. You'll read, and I used to quote, articles explaining how the pH scale is logarithmic-- pH 8 is ten times higher than pH 7, pH 6 is ten times lower than pH 7-- and based on this some reason (and I used to accept) that just a few tenths change stresses out your fish. Turns out there's been a lot of study and literature on effects of sudden pH changes on fish and it's something of a myth:
technical memorandum
Although it was once believed that fish could not tolerate sudden pH changes, studies conducted by Brown and Jewell (1926) and Wiebe (1931) showed that certain fish species could tolerate such rapid changes, within the normal pH range.
This work demonstrated that the fish species studied tolerated rapid pH changes of 1.3 to 3.5 units when these changes occurred within the physiological-tolerance pH range. When the pH changed to a value that approached the species’ normal upper tolerance level (i.e., 9.0) or
exceeded their upper tolerance limit (9.5 and 10.0), mortality occurred (Witschi and Ziebell 1979). Based on findings from these studies and personal communications with CDFG fish pathologists (Modin, pers. comm., 1998), it is concluded that neither acute mortality nor chronic
sub-lethal effects would be expected in fish experiencing rapid pH changes when all pH levels to which fish are exposed remain within the range of 6.5 to 8.5.
The ability of fish to rapidly acclimate to waters having substantially different pH values is further demonstrated by hatchery stocking programs and the freshwater tropical fish (aquarium) industry, where it is common to move fish from one water body or aquarium to another that differ by at least 0.5 pH units, and often by more than 1.0 pH unit. However, it should be noted that this “stocking” of fish typically involves waters with pH values in the range of 6.5 to 8.5 units so that the fish are transferred to waters with pH values well within the range that is physiologically acceptable to them.
Some of the study was done to determine the effects of acid rain on native fish and some done to determine the safety of stocking hatchery raised fish (like salmon) into wild waterways of varying pH. All of this varies somewhat by species, so for me this doesn't mean throw all caution to the winds with pH changes. Probably as a legacy of former beliefs on this, I'm still somewhat conservative in this area. Without trying to write a book on this-- there's more to the subject, including the thoughts of some on the effect of dramatic changes to hardness-- some experimentation after learning this taught me that none of my fish are bothered by changes of several tenths pH between 7 and 8.
Bottom line of why I've gone into all this is the frequently stated notion that "exact pH is not that critical as long as it doesn't fluctuate", implying pH needs to stay rock steady or else you're stressing your fish. Not true, at least within reasonable limits.
