Ridiculously high pH

[Mvjnz]

I just testet the pH in my pond, and it would be mostly rainwater. I occasionally top it up with tap water, which has a pH of 7.8, but for the most part I let the rain top it up.

The pH came out ridiculously high, as seen in the photo. What's going on??

[Dave66]

Something in the pond is hardening the water, driving the pH up. If there's limestone around your area, that's your source. Sedimentary rocks will always harden water. Even rain runoff over sedimentary rocks can harden your pond water.

Dave

[Mvjnz]

Doubt it. The pond is lined, so it's not in contact with any dirt. When it rains it floods, but the garden is just soil and clay anyway. No sedimentary rock as far as I know, and we did dig a big hole in the garden for the pond, and all we saw was clay.

[Dave66]

Clay, depending the type, can harden the water as well. The upshot is something in your pond is leaching calcium carbonate, hardening your water, thus driving your pH through the roof.
Occurred to me that the rocks around the border of the pond (to hold down the liner and for decoration) could be suspect. Way to check is take a rock, pour a little white vinegar on it. If the rock fizzes, that's a hardness source.
Of course, thoroughly rinse the rock afterwards 'til you can't smell the vinegar.

Dave

[Mvjnz]

There are some sandstones around the edges... I'll test them.

Thanks:)

Strangely enough the fish don't seem bothered. I have 3 goldfish and 8-9 zebra danios, and I'm always finding danio babies, and they all seem fine...