A pond's pH, or level of acid, should be right around 7 on the pH scale, or neutral. The higher the acidity in a pond, the lower the pH. If you want to lower the pH, you must add acidic substances to the pond, or the surrounding soil if the pond is unlined. When changing pH in a pond, consider the existing animal and fish inhabitants, as well as the plants in and near the pond.
Control the pond system. If you have a lined or cement pond, it is more likely that you can fix the problem quickly, since the acid you apply will stay in the pond and not leach into the soil. If your pond is bare soil bottom, or unlined, consider starting fresh with a rubber pond liner.
Check the soil. If lining the pond is not an option, have a soil test done on the shore to see what the soil pH is. If it is highly alkaline (high pH) this may be your problem. Any acid treatment you use on the pond is buffered by the soil, which has a near infinite buffering capacity: it will keep readjusting the pond's pH to match its own. You can amend the soil by adding acidic sphagnum peat moss, or commercial acid mixes.
Adding vinegar may solve some minor alkalinity problems. Use the household strength vinegar, not cooking vinegar, which is less acidic. Realize that any fish or plants in the pond will not adapt well to this or other acid treatments, however. You can repeat this treatment for a month or so before trying other means, if needed.
Get ready to acid wash the pond. If vinegar doesn't work after a month, you may need to perform an acid wash of your pond. Consult a pond and pool company to select the best kind for your pond; the choice depends on the pond materials, depth and size. This process requires you to drain the pond, safely storing fish and plants, then washing down the pond with acid.
Clean the pond. Muriatic acid, also called pond acid, should usually be diluted for pond use. Always add the acid to the water, not the other way around. Keep in mind this is a form of hydrochloric acid and very strong, so do not allow children or animals to observe or be nearby. Wear old clothes, rubber gloves and boots, and scrub down the interior of the bond with acid and a scrubbing brush.
Refill the pond with half old, half new water, and condition the water with dechlorinator and any other additive you feel is needed for your specific fish and plants. Let it sit for at least a day before re-adding fish and plants; longer if there is a chlorine smell about the pond.