In order to determine how important pH can be to our ponds, we need to first remember what pH is. This means we have to go all the way back to high school chemistry class. Stick with me for a minute...I promise it will not be that painful!
According to the Carlsberg Laboratory, pH stands for the power of hydrogen. It is basically a scale from 0 to 14 that measures the acidity or basicity level of an aqueous solution (a liquid with water as its base). Pure water is said to be neutral, having a pH of 7. A solution with a pH of less than 7 is said to be acidic while a solution with a pH greater than 7 is said to be basic (or alkaline). A number of things can raise or lower the overall pH of your garden pond or Koi pond. Fresh rain water can alter pH as can the plants you have in your water. That's right, I said even the plants living and breathing in your pond will alter its pH. When carbon dioxide is made in the water by plants, it converts into carbonic acid and this brings down the pH into the acid range. There is good news though! If you have rocks or cement that your pond water is interacting with your pH should be balancing fairly well. Limestone (calcium carbonate and magnesium carbonate), clam/oyster shells and grouting along your ponds edge all impart "carbonates" into your pond water, raising its pH levels.
As it relates to garden ponds, a safe range for your flora or fauna would be anywhere between 7.0 and 7.5. In order to test the pH level in your garden pond, I would recommend picking up a standard pool testing kit for $5 or so at your local "sell everything" store. They are very easy to use and last a pretty long time.
Usually, a pond owner shouldn't need to do anything to their water to maintain a stable pH. Remember water is naturally neutral. There are some regions where adjusting your pH may be necessary. If you find you have a lot of limestone in the ground around you, this may be a concern. If adjustment is necessary, remember, only adjust your pH levels in your pond about .2 units per day. Sudden changes in your ponds pH could result in the illness of even the death of your plants and fish.
Another important point to remember is that low pH (pond water acidity) will kill everything in your pond, high pH (pond water alkalinity) will mostly just make them uncomfortable. Symptoms of low pH in your water include (but are not limited to); "slimier than usual" fish skin, overall lethargy, clamped fins and surface piping (breathing at the surface). I recommend you keep a box of baking soda on hand for emergencies. It is just sodium bicarbonate, so it should not injure your fish if applied properly, but it will increase the pH levels of you garden pond very quickly, preventing a huge catastrophe.
Also remember the easiest method of maintaining your ponds pH is to keep it as clean as possible. Remove the leaves and sticks as they fall in to limit debris breakdown and keep your water circulating!
