There are three purposes to lime ponds:
- Increase the availability of nutrients
- Increase pH and the capacity to buffer against pH fluctuations
- Sterilize ponds before stocking
Nutrient Availability
Phytoplankton are the base of the food pyramid in fish ponds, and are especially essential for the growth of small fish. Phytoplankton need phosphorus to grow, and nutrient levels in ponds are based on the soil type where ponds are found. 
Phosphorus is almost always the limiting nutrient in freshwater ponds, and is even less available when the water is acidic. Limestone application (calcite or dolomite) increases the pH of the water, impacting the pH of the soil, and releases phosphorus.
pH Increase and Buffering Capacity
A pH of 6 to 9 in considered desirable for most freshwater fish. In acid soils, commonly found throughout Florida, ponds typically have low total alkalinity, total hardness, and pH. Adding lime increases the pond pH, as well as total alkalinity (the carbonate and bicarbonate concentration) and total hardness (combination of calcium and magnesium). For these purposes, only use calcitic (calcium carbonate) or dolomitic (calcium and magnesium carbonates).
Aquatic plants impact the ability of a pond to buffer against daily pH fluctuations by producing carbon dioxide as a by-product of plant respiration. Carbon dioxide acts as an acid in water, and accumulates at night, lowering the pH. During the day, plant photosynthesis consumes the free carbon dioxide, increasing the pH. Increasing total alkalinity increases the amount of carbonates to bind with the free carbon dioxide, increasing the ability of the water to buffer against pH fluctuations.
Liming pond by pressure spraying lime from a barge. Photo courtesy of Auburn University.
Lime for Pond Sterilization
A third type of lime, Hydrated Lime, is used in pond management to clean commercial fish ponds before restocking. Hydrated lime is calcium hydroxide and temporarily raises the pH of mud and water quickly and dramatically to kill diseases and pests that may impact the next crop. This type of lime is almost always lethal to fish, and should only be applied to drained and cleaned ponds.
The general rule is 500 pounds of hydrated lime per acre of pond. For example, if a 90 foot by 40 foot pond needs to be treated, the calculation would be as follows:
90ft x 40ft = 3,600 square feet
1 acre (A) = 43,560 square feet
3,600 / 43,560 = 0.082 A x 500lb/A = 41.32 pounds of hydrated lime
Hydrated lime is a strong base and special care should be taken when applying this substance to ponds to avoid any contact with skin or eyes or inhalation of dust.
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