The pH in my pond water is very high (often around 10 when I measure at midday). The pond is a mix of rainwater and the leached nutrient solution from my hydroponic greenhouse vegetables, which I reapply to other crops in containers.
Don't add acid into the pond. Instead, add acid into the irrigation water with an injector at the time when the solution is pumped out of the pond onto crops. This approach is more likely to control pH than adding acid into the pond when pH will naturally fluctuate.
Don't add chlorine into the pond. Chlorinating the pond is also unlikely to reduce algae in the presence of high nutrient levels, sunlight, and high pH. Injecting chlorine after the water is pumped from the pond and irrigation water is also acidified is most likely to be effective.
Don't be too concerned about the high pH. The pond's pH will increase during the day because algae remove carbon dioxide from the water during photosynthesis in the presence of sunlight. Carbon dioxide (CO2) is acidic in water. Algae and microbes tend to drop pH in ponds during the night as they respire and add CO2 back into the water.
Send a pond water sample to a lab for nutrient testing. High pH in the pond can cause micronutrients to precipitate out (become insoluble). As algae and microbes grow in the nutrient solution, the nutrient balance may change - so it will be useful to know what nutrients are actually dissolved in the water. That can help you decide if any extra nutrients should be injected back into the solution when reusing this water.
A diagnostic test of the pond water can also measure alkalinity (think of this as dissolved limestone). High water alkalinity causes more issues than high water pH if the pond water is used for irrigation. Highly alkaline water has the greatest effect on increasing substrate-pH of irrigated crops over time.
If practical, you could cover the pond to reduce algae growth and pH fluctuations.
You may be interested in these other associated items:
