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Climbing Pond pH

What can be done about high pH in ponds?

By Stephen M. Meyer |

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Q. I have just established a backyard pond and it is now one month old. When my pond was newly filled, the water's pH reading was normal. Now, however, my readings are very high, about 8.4 to 8.6. I have this idea that it has something to do with the high levels of pollen. What can I do? Can my goldfish tolerate this condition? They seem to be doing okay for now.

A. First, let me congratulate you on good pondkeeping skills. The fact that you measured the pH upon setting up the backyard pond and have monitored it since then puts you in the top 1 percent of pondkeepers in my book. You are quite right to question the unusual change in pH and to wonder about its effect on your goldfish.

Your suspicion that the pH change is at least partially related to plants is correct, though not pollen. In your part of the country, water tends to come out of the tap at a pH between 6.8 and 7.2. This is what I presume you meant by "normal." (There is really no such thing as a normal pH, so you should always provide the actual numeric reading.)

Fresh from the tap, the water has a fairly high content of carbon dioxide (under pressure) that, when dissolved in the water, forms a weak acid. Thus, the pH is artificially low. When the water is allowed to sit in the open air, a considerable amount of the dissolved carbon dioxide dissipates. Correspondingly, the water's pH rises.

The second cause of pH rise in new backyard ponds is algae. I will bet you noticed that your pond water began to turn green after a week or two. That green coloring is planktonic algae. Algae, like all green plants, converts sunlight to food via photosynthesis. As part of that process, the algae remove carbon dioxide from the water and produce oxygen. Again, removing carbon dioxide from the water produces a rise in pH.

If you measured the pH during the day when the algae was busily sucking up carbon dioxide from the water, it is not surprising that the pH was so high. Had you measured the pH about 4 a.m. before the sun rose, you would notice the pH was around 7.5. This change in pH occurs because at night the algae switch to respiration and dump carbon dioxide back in the water, acidifying it. So your backyard pond pH oscillates over the course of a day between 7.5 and 8.5.

Your goldfish are pretty adaptable, and these daily swings in pH occur gradually. However, it would be wise to have a small circulating pump in the backyard pond to aerate the water. Even better would be a waterfall or fountain to really aerate the water. Keep it running 24 hours a day.

This will reduce somewhat the shifts in carbon dioxide dissolved in the water, thereby reducing the size of the pH swings. It will also stabilize the oxygen levels.

As the algae die off, the pH will come down. Whatever you do, do not try to reduce the pH using pH-reducing chemicals, acids and the like. In backyard ponds this often causes a pH "crash" that will kill the fish.

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Reader Comments

Jacquie    International

11/24/2014 7:14:53 AM

I have kept the same fish in my garden pond for the past four years without any problems. I do not need to use tap water to keep it topped up as we have access to natural water from which I draw every few days. The water is clear. My fish have always been healthy but this past week I am noticing some seem to have a red fungus or sores on their bodies. I have just done a PH test and it is reading more than the tester shows at 8.2. I did have a metal ornament that fell into the water a few weeks ago which I didn´t notice for a couple of days. Could this have been the problem. If so, what should I do to help the ailing fish please?

no name please    Santa Rosa, CA

10/7/2013 9:49:15 PM

High PH in my 1000 gallon pond was a problem. 8.3 before dawn and sometimes as high as 8.7 late on a hot sunny afternoon.
Santa Rosa, CA adds sodium hydroxide (lye, caustic soda and an ingredient in Drano) to adjust their pH. The pH treatment is to comply with Environmental Protection Agency (EPA) regulations on the copper content in drinking water. Raising the pH helps them minimize the leaching of copper and other metals from the distribution pipe into the drinking water.
Although our local natural water is a lovely pH 7.4 in a nearby creek, when it comes out my garden hose it is pH 8.5. So almost daily I was adding garden hose water + > 5mL hydrochloric acid to keep my pond at a pH level lower than 8.3. Sodium hydroxide reacts with hydrochloric acid and sodium chloride is formed: NaOH(aq) + HCl(aq) ? NaCl(aq) + H2O(l) A little salt is natural for goldfish, better than lye or muriatic acid alone. The salinity of the pond tests negligible, 0 ppm.
I added gallons of 6.8 pH reverse osmosis water as well. I was constantly fighting to keep the pH lower than 8.3. I tried peat tea. It made the water tea colored.
Needless to say it was wearing me out. The water eventually turned pea soup green, as our home-made bio filter with a small pump could not handle the bio-load. 2 weeks ago I invested $400 in a proper pressure filter rated at 1500 gallons with a UV light and a debris handling pump rated up to 3000 gallons and the pH is no longer going through these huge diurnal fluctuations. The water is clearing up, and I’m starting to relax. The pH is currently stabilized at 8.1.
TL, DR: If your water is green, your algae may be “breathing” the pond pH into dangerous territory.

wesley    scottsville, VA

8/12/2013 9:03:03 AM

This article helped me a lot I have had the pond up and running for months but it became so polluted that I drained all the water left plants in and than started over with new the ph is about nine during the day and eight at night.

Kari    monroe, WA

6/23/2013 11:43:31 AM

I was told to add one fourth cup of viniger daily to reach a normal ph. I have done so daily for over a week and continue to get a 8.0 reading. The gold fish seem fine and water is clear. Ive never had sick fish or a issue with them getting sick but when we bought a fish at a pond place I was told to have happy fish I needed my pond at a 6.5 reading. We have a couple of plants,a under ground filter and a wsterfall for our seven fish in a large hard shell pond and the ammonia s fine. Should I stop and leave it alone?

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