This information was first posted in our "Question of the week."
Acid rain reduces carbonates – and carbonates are the buffer (measured by KH or t.a.) that help a pond maintain a stable pH. Acid rain adds – well – acid, which is neutralized by the carbonates – so long as there are carbonates. That’s why KH (or t.a.) should always be over 100 – so that there is adequate buffering capacity, and any acid rain does not immediately end up reducing the pH or causing it to crash and kill the Koi. Acid rain lowers the pond's pH. This problem is exacerbated if extra acid rain enters the pond via runoff from nearby landscaping. Pond liners should always extend several inches above the highest water level, and runoff should be diverted away from the pond through the use of sloping pond margins and/or swales.
Does acid rain help by diluting toxins? That’s a really interesting and complicated idea. In effect, any water added to the pond volume will dilute whatever is in the pond, including toxins. If you consider Acid rain as a toxin itself, then the toxins are being increased by acid rain. In general, when we discuss toxins in a Koi pond, we use the word ‘toxin’ as synonymous with ‘poison.’ While acid rain is a concern because it reduces pH buffering, it does not itself act like a poison, so would not be strictly speaking considered a toxin.
For a more in depth discussion about avoiding a pH crash and the value of KH (or t.a.), you may want to acquire our ebook “Alkalinity and pH.” It can be found as either a pdf or ePUB here: