This is a must have if your home has city water.  City water is treated with chlorine and chloramines that can be toxic to your fish.  This product eliminates these toxic additives so the water is safe.  It should be added every time you add some new water to your pond.  Especially if changing more that 10% of your pond’s water volume.  There are similar products from other companies, but I find that this product treats the most water for the price.   Always great to have on hand.  It is available in several different sizes.