I seems like such a simple thing but it one that some pond owners put last on their list of things to do. The water inside a fish pond must be kept clean as the inhabitants and plants living inside the water depend on its cleanliness to survive.
There are a number of ways to do this so that the job becomes easier, manageable and so is put off less frequently.
You could remove your fishes from your pond and put them in a container while draining the water content of your pond completely or only partially remove the water and clean the pond while your fishes are still in it. Partial removal is recommended to introduce new water slowly. Drastic changes in temperature and ph of the water can cause your fishes to get sick.
By using a siphon or pump, take only a portion of the pond water and extract it to a bucket or onto the soil if you are outside. At this stage you can also remove decaying vegetation, other debris and fallen leaves as they are extracting valuable oxygen from the water and will end up rotting and fouling the pond.
If you have time, while the pump and filter are turned off you can check their integrity. Inspect each piece of equipment; pumps, lights, liners, filter, connections, tubing and cables.
Pumps - Look for damages like blockage, electrical shock, or oil leakage. Open the pump and inspect the impeller.
Tubes - Inspect for rupture, stiffness or to see if they are kinked. Constrictions like these can affect the flow of water in waterfalls, fountains, or filters affecting the ability of aerators to work effectively.
Pond liner - Look for punctures or small tears that need patching. If larger degradation or damages are present, replace accordingly.
Electric cords -Surface cracks or frays can cause hazardous shock that can affect both humans and fishes.
Filter - Clean this to eliminate visible debris that reduce filter effectiveness.
Once all has been checked you can begin to refill your pond. Check the temperature of the pond and see that the water coming in doesn't vary too much from that, also use a ph kit to measure the acidity of the water. Cleaning the pond needn't be a big chore if you take small steps and the longevity of your pond life will benefit.