Hydrated lime. or calcium hydroxide, is also used in fish ponds. While it will increase hardness (the calcium component), and temporarily increase the total alkalinity (the hydroxide component), its primary effect is to raise the pH of mud and water quickly and dramatically. Ponds should be drained and cleaned prior to applying this product. Enough hydrated lime should be added to cover the entire bottom with a thin layer, and then water added. The pH in the upper layer of the mud and in the water will rapidly rise to 12 or higher, a level which will kill most disease agents and/or pests. The elevated pH will last for about a week, prohibiting any stocking of fish. Always check the pH before stocking. Adding hydrated lime is not recommended when fish are present in the pond. The sudden increase in pH is often lethal. Hydrated lime (calcium hydroxide) is an inexpensive and effective pond sterilizer, raising the pH quickly and dramatically above tolerable levels for most aquatic organisms. It should be used carefully, avoiding contact to the applicator, and never used in ponds containing desirable fish.