PH, KH, GH 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 )differs from my aquarium, tap water, and my aged water (

[FrozenFins]

Aged aqaurium water in my opinion is close to pointless. Go out to your lfs or pet store and get a water conditioner, it will make the water safe instantly for fish. Water conditioners are like 10$. I wouldn't worry to much about gh unless your keeping invertibrates who need that calcium in the water for there molts and shells.

 

[fhsplyusa]

I store my water for the purpose of heating and ease of water change for my tanks in the garage, not for aging. I am also not worried about a specific GH, rather the significant difference in GH from my tap/aged water to my aquarium water. Same as with the PH. 

[CT_]

tap can absorb co2 from the air and get more acidic as it ages.  the jump in gh must be something in your tank disolving calcium or magnesium into your water.  What substrate do you use?  do you have any rocks that may be disolving?  do you add things like wondershell?

Edited February 22, 2021 by CT_

[Philip]

Camper is correct that CO2 absorption will drop the pH. I doubt it will be almost a whole pH unit over a short period of time like you have, but the surface area might play a factor. 

As a water chemist, I can tell you that while pH is very important, in and of itself it's sort of meaningless. Only taking it into account as a whole with your other parameters can it tell you something. 

For example; orange juice has a pH around 2 and so does Sulfuric Acid. One I enjoy with a bagel in the morning, the other would ruin my day if I drank it. 

Get rid of any chlorine, see where your hardness is at. Watch ammonia, nitrate and nitrites. I also like to check conductivity and don't let the tank cycle up.  

You'll be fine. Don't mess with what you have too much or it will swing all over and you won't be able to control it. 

[Frank]

If you do a 10% water change every other day, over two weeks, it would average out to the same as two 30% water changes.

That might be a thing to try.