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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

Kevin    International

11/7/2015 10:14:24 PM

I have 300lt pond and had 13 fish, all the gold ones have died for no visible reason. The 3 black ones and 2 Shabunkins to date have not been effected by what ever is killing them.There are two different types of plant life in the pond,a tall thick stemmed reed which flowers once a year with a flower similar to an Iris. The other is an Elephant ear palm which grows quite well sitting in the pond water. The pond does have a 24 hr running fountain circulating the water which is topped up by tap water. The PH in a test tube is always high and blue in color and when lowered with PH down returns back to the blue color by the next day. There is the normal fine algae on the side of the pond but this has been there forever. The fish have been in the pond for a couple of years with no problems, this current problem of dead fish has started only in the last two weeks.The fish have been checked for parasites and fungus and non were found. The PH level has not been a problem in the past and I'm not sure that it is the current problem. Do you have an answer to what it could be???

Luke    Australia, DE

9/6/2015 7:59:34 PM

Thanks Thanks Thanks !
I have also just established a backyard pond and I am having the same problems - this answer was just great and my fish (I hope) will thanks you !

Dennis    Toms River, NJ

4/19/2015 10:21:25 AM

My 5000 gal pond is very acid mid day. PH is about 6.0. I use well water. The water is also very hard and very green. First i would like to slowly raise the PH. because of its size would baking soda be OK? I know to raise it very slowly. Also of note I recently fertilized my potted aquatic plants and i did bury and cover the plants food tabs. Reason I am questioning and concerns about my well established pond is that the fish seem to be porpoising more than normal but not gasping for air. Yearly reproduction has been very good, and actually too good since it is heavily populated. I have a waterfall and it is aerated as well. I am very familiar with disease and parasites and there is no sign of any

Cathy    Long Beach, MS

4/4/2015 8:46:02 PM

I have a fish pond that has black lining and a pump that is hooked up to a 2 gallon bucket with a air condition filter inside to catch the algae. I also have a hose from the bucket to make a waterfall. I have well water. I am having a severe problem with the algae. Any suggestions

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