You have a pond that has become too acidic, from acid rain, for survival of your fish. What kind of?

Do you know what pH the fish prefer? Most fish are quite hardy and will adapt to a pH that's not ideal for them. Although a sudden change in the pH will be a shock to them and may even kill them. Whatever you do, do it slowly.

If you want to do it chemically, use a buffer. However, you can achieve the same thing with an RO (reverse osmosis) system, or even just peat or driftwood.