Tom Barr...question re: "KH is OK, but PH is waaaayyyy low...help?!?"

Too much CO2 or.........bad pH test kit.

Measurements, test kit etc?

Regards,
Tom Barr

He's a man of few words............your fish are not stressed........ what does that leave?
Len

Did you test your tap water fresh from the tap or aged/aerated? If the former, the test is meaningless - you need an off-gassed baseline for tap.

Did you compare the KH of the two, and are you using current tank KH for your calculation of CO2?

You said in your original post the the pH was, "6.0 - 6.4" and the kH was 5.5. I split the difference.....6.2 @ 5.5 kH = 104.108ppm.
The fish are fine.
One of three things.....bad test kit, mis-read by tester(I don't think so), something in that tank is causing an inaccurate acidic reading.
Len

The water coming out of your tap has been under pressure and has probably been quite a bit colder than it is just now. That means that it had a much higher capacity for holding gasses in solution, gasses like CO2 which will drop pH. If you let it sit for a while at room or tank temps (when it has a lower carrying capacity) the water will dump gas into the atmosphere -- it "outgasses".

If you fill up a change bucket with fresh tap and stick your hand and forearm in you'll notice little bubbles forming on your skin, collecting on hairs. Thats outgassing. Aeration will speed the process up, as will giving it a good stir every little bit.

If you test for pH straight out of the tap, you're probably testing water that is unsustainably supersaturated with CO2, and therefore has an unusually low pH. After the water outgasses the pH reading will be truer (and higher).

The carbonate content of the water doesn't outgas, so KH should be the same straight away or the next day.