PH, GH, KH questions

[FHSPLYUSA]

I am trying to figure out why my PH, KH, and GH differs from my aquarium, tap water, and my aged water (45 gallon Brute trash can).

Tap:
PH - 8.2
KH - 6
GH - 3

Aged water:
PH - 7.5
KH - 6
GH - 3

Aquarium:
PH - 8.2
KH - 6
GH - 14

My tap and aquarium PH are both 8.2, however the PH of my aged water is 7.5. The GH in my tap and aged water are both 3, but it is 14 in my aquarium. The KH is the same across all 3. I did all the tests using a API test kit and did them multiple times to insure they were accurate.

I keep a Flowerhorn and noticed he gets really stressed out after water changes (30% weekly). My aged water is also heated to match the aquarium temp. He is kept in a bare bottom tank (no substrate

 

[twentyleagues]

Trapped co2 possibly. Try aerating your aged water while it's aging see if that affects the readings. You'd think that the process of getting it out if the tap would do that also but maybe not. Temp will affect those readings also to a point. Well or city treated? If the reading are correct then it's the only thing I can think of. I'd also try and aerate your tap water. The gh change could be causing the stress or it could be simply the waterchange process causing the stress. Has this changed recently? Or has it always been like this?

 

[duanes]

When you say 30% weekly, does this mean one 30% water change per week?
Although pH are GH/KH are sometimes related, ie a high pH usually suggests harder water.... that it is not always the case.
A GH of 3 (to me) is not hard water, and therefore, low in buffering capacity (alkalinity)
Could you test for that (Alk) and post results? (Tap, tank before a water change, and the test after a water change)
Alkalinity tests are usually available at pool stores, hardware etc.
The pH drop in "aged water" suggests a slightly lower buffering capacity.
Are you running anything in the filter that would help buffer the tank? such as aragonite (crushed coral), sea shells, stones?
When you measure pH, do you check right before a water change, and then after?
Please do this, and post results?
I'm curious if they are drastically different before and after.
If so , this might account for the stress after.
If pH, and alkalinity are drastically different after, this would suggest to me, more water changes are needed during the week to maintain a stable pH (equilibrium).
And because your water is soft, this also suggests a low buffering capacity, and may account for the stress after.

 

[Rocksor]

Where there any decorations in the main tank before? Did you medicate the tank before?

I don't understand how your GH will go up in the main tank after a water change with aged water. Something must be going into the tank to increase the GH (a measure of calcium and magnesium)