I am guessing that the lime added was at a "per acre rate" to bring it to X pH? If so, that rate is for roughly a year. Plus, if it was just added to the water, and not your watershed, more acidic water enters on every rain event to quickly counter a water only ag lime application.
Some may not agree on my thoughts for ag lime, but I add about 5-10 times what a soil sample of the watershed and pond calls for to bring pH to 7.4.
Ag lime (crushed Dolomite limestone) will have a maximum pH of 8.2-8.4, which is still very safe for fish, and an application will last 10-15 years before a repeat is needed. Also spread the ag lime at whatever rate you choose, not only into the pond, but also in as much of the watershed as possible.
