For application rates of alum, you can have your local pharmacist measure some alum into one gram increments. Then collect a one liter sample of your pond water--ideally getting a little from several different areas of the pond. It is probably wise to obtain your pond water early in the morning when pH levels are already low from plant respiration. Obtain an accurate pH measuring device and incrementally mix the alum into the water while measuring pH change. When reaching desired safe pH levels you now have a g/L or ppm value for application rate. Now calculate number of liters of water in your pond to estimate safe number of grams for total application. You probably don't want your pH to go under 6.5. If, for example, your estimated application value is 100 kg you could theoretically apply 80 kg first and check pH values after a couple of days before applying the last 20 kg to build in a safety net. If your starting pH is above 9 to start, you may get the added benefit of reducing your amount of unionized ammonia, which is toxic to fish. The alum will also bind with available phosphorous to yield aluminum phosphate which will settle to the pond bottom. Clarity will improve because fewer nutrients will be available for aquatic plants to use. A company known as General Chemical in New Jersey can help you get large quantities of alum possibly for a lesser price.
