I went and did some searching for you on this, I found these two articles I thought might be of intrest. Both are copied from well know Koi farm/store sites. They are directly copied and have not been altered. If you would like I can try and send direct links to them but so as not to post advertisement I copied and pasted them. Hope this answers some questions. Larkin
BTW, you will notice some information is repated, and there are two distinct choices for adjusting.
#1 - The pH range of 6 to 8.5 is acceptable for most pond life. The primary concern with pH is its direct relationship to the toxicity of ammonia and nitrite. Each pH interger above the neutral 7 reflects a tenfold increase in such toxicities.
Any pH value below the neutral 7 is considered acidic. Baking soda or ground limestone will raise the pH level.
pH values over 8.5 will definitely stress the fish to the point of disease. We receive some calls where the pond owner claims to have a pH reading of 9.0 or more, and the cause for this is cement or mortar leeching toxic lime into the water. Bricks or untreated mortar blocks used as plant pedestals, run-off water, and reconstituted materials used in or around the pond may be washing in leeched lime which can cause severely high pH readings.
We suggest you use a commercial lime neutralizer or a nontoxic pool sealant paint if you have a concrete pond.
If the pH is normally high in your tap water, we suggest obtaining more suitable fish for your pond, such as the common goldfish, shubunkin, or comet goldfish. These fish can tolerate higher (non-toxic) pH levels.
Lowering pH in a fish pond is more difficult and can be achieved by adding peat or vinegar.The pH of a pond fluctuates. It's at its highest in the afternoon and lowest in the morning. When the pond water's pH reading is below 7.0 the water is too acidic and is harmful to both plants and fish. Simply raising the pH is only a temporary fix to the problem. It needs to be stable. To correct the problem raise the pH to a point between 7.2 and 7.8 then stabilize it.
#2- Ph is a measurement of whether the water is acidic of alkaline. Ph is measured on a scale ranging from 1 to 14 with 7.0 being neutral. Any measurement below 7.0 is considered acidic and any measurement above 7.0 is alkaline. The ideal ph for koi fish is 7.4 but koi will do quite well in water ranging from 7.0 to 9.0 as long as it does not fluctuate too much. Imagine you have a concrete pond that normally has a ph of 8.5 and you are constantly adding acid to the pond to lower the ph to 7.4. If each week the Ph bounces back up toward 8.5 again, you are doing much more harm to your koi than any possible good that might be gained by getting the ph to stay at 7.4. Koi fish do not do well with constant change so leave it alone. What you do need to watch for is a “ph crash”. A “ph crash” is when in a relatively short period of time the ph begins to drop and it won’t stop until it reaches 5.5. This can happen literally “overnight” in a small pond. At a ph of 5.5, koi will begin to die within a few days. If this happens you can easily raise the ph by simply adding baking soda to the water. Try adding one cup for every 1000 gallons and check it every two hours until the ph is back to at least 7.0. Anytime all the fish in the pond suddenly begin to act differently at the same time you should suspect a ph crash. You can lower the ph in a pond by adding acid. Muriatic acid (pool acid) can be used, but use it very sparingly because a little goes a long way when used to lower the ph.
Re: High PH level
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