Question
QUESTION: All tests on my pond came out normal except the pH which read around 10 (eye-balling the color chart). Can I use distilled vinegar to increase the acidity, and how much for a 14,500 gal pond?

ANSWER: A pH of 10 is too high as you've surmised.  An ideal pH is 7 to 8 but up to 9 can be ok.  Vinegar can indeed lower the pH but not as easily as an inorganic acid like hydrochloric acid (called muriatic acid at pool stores).  I have some pH down solution sold to lower the pH.  It has sulfuric acid, an even stronger acid.  How much vinegar you would need to add to your pond to get the pH below 9 would be a total guess on my part. There are too many factors aside from the initial pond pH and pond volume.  Hardness and alkalinity will both raise the pH and also buffer it (holding the pH up even with more acid).  You should test for hardness and alkalinity.  They are probably high.  I suggest testing your tap water's pH.  If it's normal, then the pH in your pond is going up for some reason.  Does the pond have any cement, mortar, limestone, rocks, etc. in or around it that could be raising the pH and hardness?  I would check into that.  The pH down solution may cost more but it would be easier to dose (directions on the bottle).  If you're going to use the vinegar (acetic acid, an organic acid), I would start with a cup of it mixed in a bucket of pond water.  Mix that and pour it around the pond.  Re-check the pH in a day to see how much it's gone down.  Good luck!

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

QUESTION: Thank you for your quick response. I tested my tap water and it shows similar results as my pond. After mailing you today I went out and bought some pH Down. I understand it may take weeks to see a change? The label doesn't tell me how often I should check and continue adding. Any suggestions? The label says safe for animals, I have 2 dogs who drink from the pond, I need not worry about them, right?

Answer
Aha!  So, we know the problem is not with or in the pond but the source water or the pH test kit.  To rule out the test kit, I suggest testing the water with two or three different types of pH test kits.  You will find they do not all agree.  So, assuming your tap water has a pH of 10, that's not good for the pond or any animals for that matter.  You may want to add a water softener to your water system.  Do you get hardness rings in your sinks and toilet?  It wouldn't be suprising.  Hard water is pretty common with a higher pH.  It's good for hard water fish like cichlids but not for your pond.  You can try the pH down but, if the water is buffered, it may take a lot of acid to get the pH down.  A better option might have been to truck in water initially to fill the pond.  Of course, when you do water changes or additions to top off the pond, the hardness and pH would again go up.  Try to get the pH to 9 or below, and the animals should at least survive.  Down to a pH of 8 would be great.  For aquariums, reverse osmosis systems work great to reduce hardness in tap water.  For a pond though, it would require such a system to make and store water almost constantly since it's a slow process.  

Once you add pH down, and the acid is mixed into the pond, the pH should go down right away.  I say to wait a day to re-test the pH because it has to mix in.  Fast pH changes are dangerous for fish as well so don't alter it too quickly.  I would add some, test in 24 hours, add more if needed, test in another 24 hours, etc.  pH down is just sulfuric acid.  Once diluted into the pond, as long as the pH is above 7, it won't harm the dogs to drink it.  Just don't let them near the concentrated acid (pH down solution).