Tank pH higher than tap

[k0olmini]

I tested the ph of my 40 breeder twice and both results came back as 8.0. I tested my tap water and it came out to 7.4. I am not running any crushed coral, it’s fairly well planted with a bunch of pogostemon stellatus octopus and duckweed (that I can’t get rid of). It’s stocked with 15 blue cochu tetras, 8 cheery barbs, 8 glowlight danios, 2 red lizard tails cats, 1 honey gourami, 1 peacock gudgeon, 6 bronze corys. I am not running any crushed coral, I have some driftwood in there too. My substrate is innate. The tank has been running for a year already. I would assume that it would get more acidic. 
 

Can anyone think of a possible cause? The only other thing I can think of is the rocks I am using. They were rocks I picked up from a Home Depot. I tested them with vinegar but I didn’t see any bubbles. Other than the rocks is there any other explanation for the ph increase? 

Edited August 28, 2021 by k0olmini

[Isaac M]

It is often very helpful to have a picture of the aquarium to see if there is anything else in the aquarium that would increase your pH.

Depending on the source of your water, if it has elevated levels of carbon dioxide coming out of the tap, then the pH will be lower. In aquariums, water is usually aerated with airstones through water surface agitation and this will cause gas exchange to where carbon dioxide levels will be less than from the tap. 

If you are topping off the aquarium and not doing water changes very often, you can also have a build-up of minerals that could cause kH/ pH to increase as well. 

I cannot really think of more off the top of my head but hopefully others can chime in with other possible scenarios. 

[k0olmini]

Thank you so much! I’m doing water changes. You bring up a really good point about the water originally having CO2 then airated in the tank