Hi Koifishgirl, as long as you don't have ammonia in your water, a high pH value is not too dangerous. Koļ can live between 5 and 10 but the ideal value is between 6.8 and 7.5. If your ph is high, ammonia can be really toxic.
Anyway if you change your pH too fast, that is a real stress for the koļ, you should not change more then 0.5 degree per day !! so don't change it too fast.
To bring the pH down you can proceed to water change with ground water, source water or better, rain water.
If you want to use acid, add hydrochloric acid , you should find that in a pharmacy, I don't know about vinager wich is acetic acid.
Maybe a bad white acid wine will enjoy your koļ too ..

Marco