What causes high pH?

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Mar 1, 2023
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Houston, TX
Tested my pond water today. At about 8.4 pH. Not sure why though. Test was taken after a 35% water change. Same water I use for my other 5 aquariums.
My aquariums run around 7.2 - 7.6 pH with regular weekly water changes. I haven't done a water change on the pond in about a month though, only top offs.
All other tests show 0ppm (Ammonia, Nitrites, Nitrates). Tadpoles have been infesting the ponds for about a month now. Fresh tadpoles about a week ago.

There's a few large tadpoles, almost frogs, I saw today.
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Test the tap water for pH.

The pH can be affected by lots of plants or algae. When they get light they produce oxygen (O2) and use carbon dioxide (CO2). If there's no surface turbulence and lots of plants, the pH can go up rapidly during the day and come down at night.

If the pond is concrete and hasn't been sealed, then it can cause the pH to go up.

If the water has lots of minerals in or limestone, shells or dead coral rubble, that will raise the pH.
Tap water is fine as I use it in all my other aquaruims and don't have this high of pH. Unless you mean at this specific point in time the tap might be higher pH then normal? I have some hornwort that has been in there for a while and the bottom portion appears faded in color in comparisison to the top of it. The pond is 40 Gal. Preformed Pond Liner

There's some normal gravel aquarium subtrate in it which was added last summer. The bottom most substrate has been in there for at least two years which is CaribSea Eco-Complete

There's a couple other potted plants but they are thriving and not looking bad at all.
Louisiana Iris Bog Plant - Dixie Deb & Water Poppy
So because it was killing me without knowing. I tested pH on tap water and it's 7.2.

Also, as far as surface turbulance. I have 3 different bubblers, one is a foam filter. The other two just bubblers.

I don't have a great overall pic of the pond atm but will get one tomorrow and post it.
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So, now totally interesting, and as you said, during the night the pH goes down. It currently tests at a 7.6 and it's 11:21PM. So I need to turn up the bubblers and filter?

EDIT: I did add 4ml API pH Down after getting the high pH reading earlier today, but tested after and still had a high reading. It wasn't until I tested later in the night that the reading was lower. Will test tomorrow during day around same time to see what it reads then.
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Don't add pH up or pH down.

The best way to prevent it fluctuating due to plants photosynthesising, is to increase aeration/ surface turbulence. Then you will prevent excess oxygen building up and reduced carbon dioxide levels from occurring.

The hornwort is normally pale on the underside due to lack of light.

The best time to test is first thing in the morning before the sun comes up and in the afternoon when the sun is just about to stop shining on the pond. Then you will see a big difference in pH if it's caused by plants.
Should I even worry about the pH? Seems like a normal thing here. I don't have fish in the pond. I will up the bubblers strength to help.
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Should I even worry about the pH? Seems like a normal thing here. I don't have fish in the pond. I will up the bubblers strength to help.
View attachment 318878
A very nice pond!

I had a problem with pH in my 55G tank full of plants, as the pH fluctuation between day and night was around 0.6 (perhaps 0.6 was not worth worrying about).

I’ve since reduced water agitation during the day by using a small HOB, and use a timer to switch on a larger HOB at night. That lowers the pH differences between day and night.

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