pH testing for lower ranges

[zendog]

I have some tanks with mostly rain water and I know the pH could easily crash, but I don't really have a great way of testing lower pH values. I'm okay with it going below 6 for some of my cories if I can monitor it. I bought one of these digital testers:
https://www.amazon.com/Newdy-Aquarium-Hydroponics-0-00-14-00-Measurement/dp/B07217NV2Q/
When I tested with it yesterday, 2 of my tanks showed a pH in the low 4s, which I doubt since the fish seem happy and are eating. I still need to calibrate it, but it appears to be all over the place, showing high 7s in the tanks with mostly tap and that seems correct. Perhaps it is my technique and I need to calibrate and make sure I'm following the instructions exactly, which I'll try this weekend.

But in the meantime, I was wondering what other folks have found to be the best way for reliably testing pH in the 4-6 range?

 

[chriscoli]

I haven't tried keeping tanks in that range, so I don't have much practical advice for you. But when I was in the lab, cheap pH meters didn't usually do well along the full pH range. And even with the expensive ones, we were always (like daily) having to do a three-point calibration to double check that the probe was still doing its job. Now, I keep a roll of pH paper on hand to double check my pH tests and as a quick check to see how things are doing. I have one roll that's wide range, and another that's a finer (but narrower) range.