Why does my pond smell?

As a general rule a pond should never really smell. If it does then it probably needs to be looked at. There are a few reasons why a pond might smell and here are some tips to help.

Some of the most common issues are a lack of aeration, decaying debris, poor water quality and sometimes when the fish (usually koi) are breeding. Dead fish, plant fragments (leaves, grass, pollen etc), dead insects, bird droppings, uneaten food are all examples of organic and inorganic matter that will create waste in the pond.

Normally all this debris is eaten by oxygen breathing bacteria. When there is not enough aeration in the pond the water can develop temperature layers, which is how the pond starts to get its smell. The top layer gets warmed by the sun and isn’t able to hold any oxygen and seals up the bottom layer with a limited amount of oxygen. When the ponds oxygen supply is depleted there is not enough of the good bacteria to deal with the waste being produced so the breakdown of the debris in the pond is left to the bacteria that doesn’t need oxygen to live in the pond. It takes a lot longer for this type of bacteria to break down and convert the waste in the pond and while it is doing this it releases Hydrogen Sulphide, Methane and Ammonia which becomes toxic to fish and smelly.  

The absolute best way to stop the pond from going stagnant and to develop a good crop of the beneficial bacteria is to have a pond pump circulating in the water to stop temperature layers from developing and to provide extra oxygen for the pond life. If you have a smaller pond an aerator which sits outside the pond with a thin 4mm tube running into the pond with an air stone can help to oxygenate the water.

Pond plants are also an effective way to help provide much needed oxygen in your pond as well as using excess waste in the pond for their growth. By doing this they help the pond create a natural balance which in turn keeps the water healthy.  

There are also products available that help to eat away at that debris or sludge in the bottom of the pond. Products such as Eco-Fix and Pondzyme.

Occasionally when fish are spawning (commonly koi) they can get pretty carried away chasing each other around and stirring up the pond. The eggs and fluids released during breeding can create a strong smell and sometimes frothy water in the pond. This usually disappears on its own after the breeding cycle is over.