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.
Since rain has no buffers, it doesn't take much to cause its pH to change. The only affect on the pond is that a small amount of buffers are consumed. 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. Happy Koi keeping.