Larry Lunsford

Every once in a while, the talk around the pond turns to the subject of acid rain. You've probably heard of acid rain. Its simply rain that is acidic (pH less than 7.0). Things like natural Carbon Dioxide and man made pollutants like Sulfur Dioxide (one of the elements of auto emissions) can produce acid when mixed with water (rain).

As Koi keepers, we are concerned about many water parameters including the pH of our pond. So we are rightly concerned when acid starts falling from the sky. But, how concerned should we be? The typical story goes like this: my pond's pH is 7.0, the acid rain is 5.0, so pond + rain = pH 6.0. From this, it would seem that the sky is really falling in on us.

Well rough calculations are ok for some situations, but this one is just too rough. Lets see if we can polish the calculations a bit and get an accurate measure of what acid rain is really doing to our ponds.

For starters, we're not mixing equal parts acid rain and pond water. Getting a full inch of rain is a fairly heavy rain in most parts of the country. Even with one inch, for most of us this only amounts to a few percent in our pond.

Acid rain is not very acidic. Of the cases I know of where people have actually measured the pH of rain, its usually 7.0 - not acidic at all. Occasionally, someone will get some rain that's a little below 7.0. I don't know of anyone actually getting rain with a pH below 6.0.

Rain water is very pure. Water that evaporates to become clouds contains very little other than pure H₂O. Rain water has almost no buffers (carbonates and bicarbonates). Since rain has no buffers, it doesn't take much to cause its pH to change.

So what happens when the acid rain meets our pond? When the lightly acidic, buffer free, acid rain meets our well buffered, neutral to alkaline, pond water the buffers quickly neutralize the acid and our pond's pH remains unchanged. The only affect on the pond is that a small amount of buffers are consumed. If you are keeping your pond buffering (also known as total alkalinity or carbonate hardness or KH) at a minimum of 90ppm to 120ppm, then you have nothing to worry about.

The only time acid rain could be a problem is if you get a large amount of it and your pond isn't well buffered. The only way your pond could suffer a pH crash is if you are about to run out of buffers. If you're low on buffers, a pH crash is imminent whether you get acid rain or not (your bio-filter is putting out a steady stream of acid, and if you have green water the algae is producing CO2 which becomes acid). If your pond is low on buffers (test using Tetra's KH kit), simply add baking soda. You can figure how much you need using this simple formula: ounces of baking soda needed = pond volume (in gallons) * desired ppm increase / 5000.

To sum it all up: keep your pond well buffered and neither the sky or your pH will be falling. Happy Koi keeping.

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