Pond balance and Water quality
Getting the right pond balance and keeping your pond water at the optimum quality is the essence of maintaining a successful pond. This requires a basic understanding of the nitrogen cycle but don't be scared it's not that complicated.
You simply need to understand that fish waste, decaying food and dead plants turn into compounds that form ammonia (reactive gas that is toxic to pond life). This is then oxidized by bacteria and turned into nitrites and then into nitrates. Nitrates are used as plant food and enter the nitrogen cycle once again. All these stages of the process (i.e. ammonia, nitrites and nitrates) are harmful to your fish but it gradually get less harmful the further down the cycle you get.
In the natural world this is going on continuously at safe levels, meaning that any new life will only form if the conditions are right as there has to be enough food in the existing cycle to support the new life.
i.e. plant dies->waste->ammonia->nitrites->nitrates->plant food->plant grows->plant dies->all starts again
fish eats plant->fish waste->ammonia->nitrites->nitrates->plant food->plant grows->fish eats plant->all starts again
With the odd bit of extra food being added gradually via other plants or insects coming in contact with the pond. But with a garden pond we humans are adding additional un-natural food (fish food pellets) into the cycle all the time, leading to a potential imbalance of the pond.
This is why we also need to take measures to help the natural cycle to counter balance the negative effect we are having. The main thing that we must do is implement some kind of filtration system to remove the organic matter such as uneaten food, decaying plants, and fish waste. Most filters also have “good” bacteria embedded into them to help create the perfectly balanced pond. There are other little things that can help keep a balanced pond such as not over feeding your fish, because at the end of the day the more you feed them the more waste they produce.
There are test kits available to check for ammonia, nitrite, and nitrates. The effects of nitrite stress can be reduced in an emergency by adding 2 1/2- 2 3/4 teaspoons of aquarium salt to every gallon of water.