How do I lower the pH in a freshwater pond?

Add a Peat ball or log or some liquid barley

That high of a pH is strange. Is there some place the water in coming in contact with some type of limestone or is the filter not working properly? The peat would be a good bandaid but I think something else is amiss.

Could there be cement or mortar leaching lime into the pond (a common cause of high pH)? If so, you may have to apply a sealant over the concrete or you’ll be fighting a losing battle.

It’s not that uncommon for the pH to be far off in the heat…how hot is it by you @ANef_is_Enuf?

I don’t see how the water could be touching any sort of limestone, no. It is close to cement, but not touching.
It’s been very hot. Mid to upper 90s all this last week, and supposed to get hotter.

Could runoff rainwater from the cement be entering the pond?

Has it rained much lately and what’s the pH of your rain tend to be?

To get it to stabilize quickly, you could use vinegar

Honestly, the heat can mess the pH up quickly. Until it stabilizes, you’re going to want to monitor a few times a day to protect your Koi.

If you can, set up some more shade around your pond during this hot spell.

@SpatzieLover GA for the shade idea, but I don’t think changing the pH quick is going to be good for the fish. They don’t like major changes in anything.

@Adirondackwannabe I agree. If it were my pond I’d make those changes over the course of the day. Quick is a relative term.

The ph was normal yesterday, and it rained last night.

If it was normal yesterday and it’s 8.4 today, it sounds like the rain introduced something into the water. Was there runoff from the concrete?

Possibly.

There are some products you can use to lower the pond pH: PondCare pH Down, for example. There are also pH stabilizers that supposedly ‘lock in’ the pH once you get it where you want it.

You might also find this discussion interesting/useful.

Does anyone happen to know if crocosmia or Stella d’oro are toxic to koi? I can’t find anything online, but we just remembered a similar thing happening in the pond last year… and that is when these flowers were blooming next to the water. Two fish died today, and they died in the same way that we lost a fish last year under similar circumstances. The pH went back down to 7.5, so maybe it was the heat. We didn’t add anything.

@ANef_is_Enuf As far as I know, the day lillies are fine.

I am not as certain with the Iris, as some types are poisonous to pond fish/Koi. Here’s a list of plants possibly toxic for Koi

I’d be concerned that it is the crocosmia, and I’d remove it if this were my pond.

I’ve found some other sites that seem to confirm the iris are not recommended near the fish.

:( So sorry to hear to of them died.

Are crocosmia related to irises? I never knew that, I always thought they were related to gladioli.

Yes, they are in the Iris family.

How about that. I learned something new. Thanks!