Question theory on pH and covers

[Bigge_al]

I measured my pH on my tank at about 7am and it was 8.4 then I removed the top to adjust decoration and left it off for a while at 11:45am I test the water again and found the pH to be 7.4-7.6. At first I thought I had messed up the test from the morning but after thinking about it realized I had to do 2 test to get the high reading. The first was the regular pH test and once I found that it was high I did the high pH test.
Is it possible that leaving the cover off the tank allowed the pH to fall by evaporation.

 

[Wy Renegade]

I would expect gas exchange to be a more likely culprit than evaporation. That said however, the difference between 8.4 and 7.4 on the pH scale is actually a factor of ten, since the pH scale is a logrithmic scale. Therefore I would guess that tester error is a far more likely source of at least a large part of the difference. Curious to hear other thoughts however.

 

[Mumma.of.two]

If I remember correctly isn't the normal pH reading in your tank 7.4? So 8.4 is the exception? (correct me if I'm wrong)

 

[Bigge_al]

I came up with another idea that I may have used a tube that was not clean from a previous test.

 

[mfdrookie516]

That's the opposite reaction when dealing with sealing off the tank. pH crashes can become an issue if you don't have good gas exchange. Sealing off the tank with glass lids can cause poor gas exchange. Usually doesn't happen, but can... and it did to me.

 

[Bigge_al]

I have 2 air walls running and the top is a standard top with a hole for the filter and heater not at all air tight.

 

[Ardynas]

I have plastic lids for my tank with a regular pH of 6.5. It tends to fall towards 6 inbetween weekly PWC's. I wonder if I remove the lids if the pH will rise? My tap pH is 7.8.