Using limestone in a garden fish pond, also referred to as liming, helps increase nutrient availability and water hardness, as well as increase and buffer the pH, which is the measure of the acidity or alkalinity. Increased nutrient availability is vital for aquatic plants, an important part in the food chain of the fish. Water hardness in ponds is measured by the amount of calcium and magnesium in the water and adequate levels are important for fish reproduction. A stable pH is a critical component of the overall health of the fish. Adding limestone to your pond keeps your fish population well fed, reproducing and in good health.
Determine if the pond will benefit from liming by testing the alkalinity of the water. Collect a sample of water from several inches below the surface in a clean quart-sized container. Do not collect sediment from the bottom of the pond, as this affects the results.
Use a swimming-pool test kit to measure the alkalinity and pH of the water sample. The pond can benefit from liming if the pH is below 8.3 and the total alkalinity is less than 20mg/L.
Purchase a bag of agricultural limestone from a garden centre. The limestone should be calcium carbonate (CaCO3) or calcium magnesium carbonate, (CaMg(CO3)2). Purchase the smallest particle size that is available to maximise reaction time and solubility.
Apply the limestone to the pond by spreading it evenly over the surface of the water. The lime sinks to the bottom of the pond and slowly dissolves over time. The application of lime generally should be repeated every three to five years.
Liming application should be completed in the fall or winter months. The recommended application amount of agricultural limestone is approximately one ounce per square foot of surface area of the pond. It is difficult to use too much agricultural limestone. The limestone will not dissolve unless conditions necessitate its presence.
Do not use calcium hydroxide, also known as hydrated lime, as it can be toxic to fish.