[Hoppy]

Like all hobbyist test kits, the pH test is only so accurate. This is primarily because of the difficulty of matching the solution color to the color chart color. But, it is also because of other combined errors. If your larger tank registers 7.6 on one test kit and 7.4 on the other test kit, it is likely that the true pH is between 7.3 and 7.7. There is no reason to care what the true pH is to any more accuracy than that. The smaller tank is more likely to be between about 6.5 and 6.9 than it is to be close to 7.4. (All test equipment is most accurate in the middle of its range, not at the ends of its range.) And, there is really no good reason to care what the pH is to any more accuracy than 6.5-6.9.

If you really feel a need to know the pH more accurately you will need to get a pH measuring probe, and at least two known, stable pH standard solutions, for calibration purposes. Measuring pH with a probe is only as accurate as your calibration of the probe.

If you were working in a chemical lab, where you really needed to know pH very accurately you would have a very expensive pH probe, and you would probably have someone who was designated as the calibration expert, who would calibrate the probe at least every week.

 

[Grumble69]

I've got the same issue. A lot of the test kits use the same indicators--bromothymol blue (regular range) and cresol red (high range). I think what we're looking for is a ph test kit that uses phenol red. It looks like it differentiates the range in the 7s a little better. These kits tend to be marketed towards pools & spas rather than aquariums.

I think I could order a bottle of phenol red from Amazon for around $5 and then use a color chart from google images to pin down what my ph is. i.e. if there is an orangish tint, then I'm probably closer to 7.2 whereas if it's pinkish then I'm closer to the 7.5 mark.

...haven't quite found the urge to do that yet though. My anal-retentive side would love to know for sure and confirm the results since it's in-between the 2 extremes of API's kit. Yet the practical side is winning out by saying it's 7ish ("that's good enough; spend your money on other things instead").

 

[Absolut Talent]

thanks for the info.

 

[Diana]

Ditto:
If the high range pH test shows mid 7s, but the lower range pH shows something in the 6s, then the low range test is right. The high range test has bottomed out. It is not accurate in that low range.

If the high range pH test shows mid 7s and the low range test also shows the mid 7s (and the numbers may not be exactly the same) then you are reading the extreme for each test: the high range test is close to bottoming out, and the low range test is close to topping out.
Call the answer whatever you like, the fish don't care. Split the difference.

Best are the other tests you have done.

GH in the right range for the livestock.
KH pretty close.

Assume the pH is in the right range for the fish, and it is a wide range, not a bulls eye on the target.