(1) PM Tank: pH 6.0, GH 4 degrees, KH 2 degrees
(2) AM Tank: pH 6.8, GH 5 degrees, KH 2 degrees
(3) Straight from Tap: pH 7.2, GH 5 degrees, KH 5 degrees
(4) Right after 25% WC: pH 6.8, GH 5 degrees, KH 5 degrees
Compare (1) and (2):
pH changes through the day, but not in the way I would expect it to. Plants use CO2 through the day, so the pH should be higher at the end of the day. Plants give off CO2 though the night, so the pH ought to be lower right before the lights come on.
Note KH is quite low, so whatever is changing the pH does not have to overcome very much buffering to do so.
Compare tap (3) to tank before water change (1 and 2):
Tap water pH is higher, but similar to the AM tank, quite a bit higher than the PM tank. This suggests that the tap water might have less CO2 than water that has reached equilibrium with the air for CO2, and this equalization is taking place in the tank. Lets see the tests of the standing tap water, @24 hours and 48 hours. This can help figure out if it is something in the tank, or purely a tap water problem.
Note the high KH in the tap compared to the tank. Something is removing the KH. Or turning it into CO2.
Compare (3) and (4):
Values ought to be in between tap and tank when you do a water change. pH is like this, but KH is not. Hmmm...
When you do a 25% water change the KH ought to be in between the 2 levels, in this case closer to the tank value, since there is still 75% of the original water in there. I would have expected 25% of 5 dKH plus 75% of 2 dKH to make the tank test about 3 dKH.
That it tests 5 dKH after the water change is a bit odd.
Here is a way to check that:
Are you using API test tube and reagent type of tests?
That test calls for 5 ml of water. Use 10 ml, and each drop you add equals 1/2 of one degree.
Are you using any other test tube and reagent type of test?
Use double the water sample, and cut the answer in half.
(I do not know how to alter the dip stick method to test like this)
The next time you do a water change use that method to test tap and tank before and after.
I have one of the cheaper TDS meters, I think I paid about $20 including shipping. Works just fine. I sure would not pay hundreds or even one hundred dollars.
Any answer from the water company about chlorine or chloramine?
How are the fish doing?