pH randomly crashed in tank with nothing in it?

[jaydawnsin]

I'm cycling my two tanks right now to prepare for fish. I have no plants, no fish, just a little bit of fish food in the bottom of the tanks to raise ammonia. I've ordered some plants but they're not here yet.

When I put the water in, the pH was 8.2 which didn't surprise me cause I thought we had liquid rock water where I live. I've let it sit for awhile, testing the water for ammonia, nitrites, and nitrates every few days. I didn't think to continue to test the pH because I haven't heard of it crashing with nothing in it. Well I tested it just now and it dropped from 8.2 to 7.2. This is day fifteen of the tank cycling, I only added fish food three days ago and did a water change last week. Any clues as to why my tank's pH would be crashing? It's a 10 gallon quarantine tank. I have yet to test the 29 gallon for pH but I imagine it's similar.

Edit: The 29 gallon is at 7.2 pH as well.

Edited August 22, 2020 by JaydenDSin
New information

[Ryan S.]

My pH out of the tap is similar to yours and my pH drops over time as well.  I've got high GH but super low KH which I think contributes to it.  I don't think it's a huge issue unless you are keeping African cichlids after cycling.  I got some crushed coral from aquarium co op to help buffer my water but I haven't put it in yet to see how it impacts things.  

[Brandy]

I think the bacteria nitrification process can contribute to acidification while they are getting established. I can't remember what article I read that makes me think that, but it was some primary research literature article--not some internet comment.  This, however, IS some internet comment, so take it with a grain of salt. My memory may be faulty. 🙂