Understanding Pond Water pH

pH Swings Cause Trouble! A Steady 7.0 – 8.5 Reading Is Our Goal

A Stabile pH … One Facet Of “Healthy” Pond Water

If you desire to get involved with keeping Koi in your garden pond, then you will need to become a “water keeper”. By this I mean if you properly maintain certain parameters in your pond water, then you will more than likely NOT experience many of the completely unnecessary problems that novice and/or unwilling-to-learn hobbyists encounter.

Koi in your garden pond are wonderful and certainly eye-candy, to say the least, but they won’t remain “eye-candy” for long if the health of the pond water is ignored. There is no getting around this issue! Koi are not pond ornaments and they deserve the same care and consideration we give to our other beloved pets!

So … What is pH?

pH is the level of acid or alkalinity of the pond water. Most of us rely on city tap water to fill our ponds. Some of us are lucky in that we have (free) well water. But no matter what the source, our water supply will have a pH measurement that will probably fall upon the slightly acidic or slightly alkaline side. This is totally normal.

As a general rule, hard water is usually alkaline and soft water is usually slightly acidic.

pH Chart for Garden Pond

A pH reading of 7.0 – 8.5 is really ideal for Koi, however they can survive in ranges between 6.0 – 9.0.

I happen to have tap water that is around 7.8, so I am lucky in that I don’t worry about the pH level when I add water to my pond. Almost all of us will find that the pH level of our municipal water will be within the “acceptable” range for Koi. And most well water, too.

Readings That Fall Above Or Below These For Too Long A Period Can Cause Irreversible Health Problems

It is essential to monitor the pH level in our garden ponds IF we really want to become responsible Koi keepers! Very affordable pH test kits can be purchased at pet or aquarium stores, or on the internet.

The tester kit I use and prefer has a liquid that you put one drop of into a little vial of collected pond water. The tester liquid changes the color of the water and by comparing it to the kit’s color chart, you can quickly tell the pH of the pond water.

We Want Stabile Pond Water pH

If You Check NOTHING ELSE … Routinely Check Garden Pond pH Levels!

Why Are Swings In pH Such A BAD Situation?

Stability of pH is what we should aim for. Rapid changes in pH can cause extreme stress to our Koi, quite similar to shock in humans.

**A sudden change of a half or more pH unit in an established pond is an indication that something happened and you need to investigate the cause.**

An established pond will normally maintain a pH reading about ½-unit above or below the pH of your tap or well water. This is fine. But swings up or down are not what you want!

Increasing pH may be an indication that lime is leaching out of concrete. Evaporation can, to a lesser degree, also cause an upswing.

Decreasing pH is primarily due to bacterial action taking place, which often is caused by lots of debris and sludge on the pond’s bottom.

While alkalinity and pH are closely connected, I am not going to address this issue here. This article is geared towards the vast majority of garden pond owners who are willing to “take care” of their pond water, but don’t want to become chemists in the process! I understand this and as I have stated elsewhere in my garden pond articles, I AM NOT suggesting in any way that I am offering 100% complete scientifically explained information on each subject.

NOTE: What I am offering here is ENOUGH important information that will arm the “average” backyard ponder and Koi owner to be successful in maintaining an aquatic environment that will support healthy Koi.

How to Handle pH Swings

If you’ve got out-of-control pH problems and there are Koi or pond fish of any type in your pond … FOLLOW the following procedures. Attempting anything else, which includes adding chemicals for pH up or down control, should be done only under EMERGENCY conditions. Attempting to lower pH chemically is not only potentially hazardous to you, but also to your bio-filter and the fish.

Pond fish and Koi do NOT like rapid pH swings. I’ve already mentioned that. So … what do you do?

pH is rising into higher levels: conduct daily water change-outs to bring the pH back into your pond’s “normal” range.

Perform a pH test after each change-out and again in 24 hours. **Also check the pH of the water you are adding as it may be part of your problem.

  • pH of 9.0  Do a daily 10% – 25% water change-out
  • pH of 10  Perform a 25% – 50% water change-out
  • pH over 10 DANGER! Remove all fish (make sure you have first established proper water parameters and such before you transfer the already stressed fish into another body of water). A massive water change-out is needed, but quite frankly you need to immediately address what made this happen!

pH is dropping: a pH that gets down to the 7.0 range or below is normally observed in a liner based or older concrete garden pond. Briefly (without going into all the scientifics) the alkalinity has been “consumed” by the biologic activity in the filtration system and the pH suddenly drops. This is referred to as a “pH crash”. So, what to do?

  • Begin water change-outs and increase aeration to the pond water
  • Add good ‘ol pure baking soda (any generic brand is quite OK and cheaper than the name brand)

How much baking soda should you add? Well … though you almost cannot overdo the addition of baking soda in emergency situations, I would start with approximately 16-ounces per 500 gallons of pond water. You don’t have to be too worried about adding too much baking soda at this point. The fish will hopefully not be any worse off than they already are.

If possible, mix it up in a large bucket of water and pour into the pond. Otherwise, dump it in! It will dissolve quick enough. Wait about one hour and then test the pH level. You will probably find the pH has climbed to around 8.0 or higher.

Once You’ve Stabilized the pH

Do NOT Stop Monitoring The pH … Keep An Eye On It!

If your Koi were removed from the pond during your pond water pH problems, they can be put back after you have stabilized the pH. I will mention that you should, by now, have determined what it was that caused the swings and have made every effort to not let this happen again.

Continue to monitor pH levels in your pond. Don’t take things for granted … check the pH once a week. The few minutes it takes to do a test is nothing compared to what you may end up going through by becoming lackadaisical. Besides the hassle of dealing with pond problems, you could end up killing all the Koi.

Don’t Be A “Fish Fixer” … Please Become A Dedicated “Water Keeper”

Speak Your Mind


What is 23 + 11 ?
Please leave these two fields as-is:
IMPORTANT! To be able to proceed, you need to solve the following simple math (so we know that you are a human) :-)