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pH problems

25 9:51:31

QUESTION: Thank you for offering this service.  I live in the Bull Mountains in Montana near Roundup.  This is Zone 3/4.  I have a 1200 gal pond with 2 waterfalls and a two lengthy runs from the waterfalls over stone to the pond.  It is lined with pond liner and uses biological filters (2).  It gets sun on some part of the pond most of the day.  I have 2 koi and 2 gold fish about 6-8 inches long.  We have had some problems with water quality over the 3 years we have had the pond.  Plants I planted last year all died, but the fish are doing great.  We fill the pond from our well, so no chlorine problems, but the water is quite alkaline (9).  I have been testing the water in the pond once or twice a day and trying to correct it with "pH down" (120ml) every other day.  As of yet no decrease in pH.  Should I use some thing like vinegar?  Or should I just keep correcting with pH down every other day?  Thanks for your help.  Laureli

ANSWER: Your pond sounds beautiful.

The best way to lower the alkalinity is with hydrochloric acid, but you need to be very careful handling the chemical and introduction to the pond's water should always be after prior dilution.  It is best to let it sit for a few minutes to allow it to react.  Vinegar is a safer solution, but it still works.  It just takes more vinegar to get the job done, meaning a good bit more money.

I would stop using the pH down.  It actually doesn't work if the problem is alkalinity-based.  It also is harmful to plants.  The alkalinity is very high, and I imagine that it has driven the pH up as well.  Just make sure to gradually lower the alkalinity, as the pH will fall during the process.  You should try for no more than .5 units of pH a day.  Also check the GH.  Fish like it between 3-6, plants like it between 4-8.  Then add 1/2 cup of rock salt or "pond salt" for each 100 gallons of pond water.

---------- FOLLOW-UP ----------

QUESTION: Thank you for responding.  I have a few follow up questions.  
1# for a 1200 gal pond/waterfall how much hydrochloric acid would you start with?  How much water would you dilute it with?  
2# If using vinegar, the same questions, how much and how much dilutent?  What does GH stand for and how do you test it?  
Thanks again for your time and effort.  Laureli

It is really hard to say a certain absolute ratio.  I would use at least 3 gallons of water per tbsp of acid and at least 3 gallons of water to one cup of vinegar.  As far as how much of either you need to use to reduce the pH is entirely dependent on your alkalinity as well as the strength of other buffering compounds in the water.  The best method to use is to start small and gradually work your way to a higher dose.  Just remember that pH, being on a logarithmic scale will take 10x as much acid to reduce the pH from 9 to 8 than in would take to reduce it from 8 to.  Along those same lines, a pH of 10 has 1,000 times the ratio of positive Hydroxyl ions to negative Hydrogen ions.  So start slow, and once you approach a pH of 7, go even slower with the dosing.  The whole time your water's pH is decreasing, it is due to the falling levels of alkalinity.  GH is "general hardness."  It is a measure of Magnesium and Calcium.  The ratio of Calcium to Magnesium helps to determine the precipitation rate of alkalinity.  If your water's GH is too high, it will make the alkalinity stay higher than it should.