When pond water stops moving regularly and becomes stagnant, the pond begins to emit an unpleasant stink. This smell is similar to rotting eggs and is due largely to decaying organic matter that is trapped on the bottom of the pond and devoid of oxygen. Smells can also come from certain types of algae that feed on nutrients released by the decaying matter. There are a few keys steps to eliminating the vile smell.
Dredge the pond. Draining all the water and then physically removing the layers of decaying matter are a definitive way to rid your property of all sources of the foul smell. The odour, however, will return if you don't address the underlying causes after you have dredged the pond. Dredging works best for small ponds installed by homeowners. All aquatic life such as fish and snails must be temporarily relocated to an area like a paddling pool before dredging the pond. You must remove the waste material by hand. Then, you must refill the pond. People with larger naturally occurring ponds may have to hire industrial equipment to complete the dredging and may find it effective to skip dredging and proceed directly with the other steps for removing odour.
Add aerobic bacteria to the water. Aerobic bacteria rely on oxygen for survival and are essential to a pond's health; they consume both algae and the deteriorating matter from plants, fish and other aquatic organisms. The top layer of water in stagnant ponds becomes heated by the sun and traps decaying matter beneath it. Aerobic bacteria eventually die off and leave behind only anaerobic bacteria. Anaerobic bacteria work more slowly at digesting matter and produce a foul smell as a by-product. You may buy bacteria to introduce to ponds, or you can find certain water filters that produce the bacteria. An alternative method of introducing positive bacteria is to sink a bale of straw in your pond.
Install an aeration system. Aeration systems add oxygen to water so that beneficial aerobic bacteria can thrive year-round. Additionally, aeration systems keep water circulating so that organic matter breaks down more quickly and does not become trapped at the bottom of the pond beneath an overheated top layer of water.