A small pond will need a complete overhaul to remove debris every five years, while large ponds need thorough cleaning every 10 years.
Start by preparing a holding tank in a shady spot for fish and deep water plants; use some pond water in the tank, unless it is particularly cloudy. Marginal (water’s edge) plants will survive as long as they are kept moist and shaded.
The easiest way to drain your pond is to use a pump, which can be rented for the day from a machinery hire shop. As the water level falls, remove fish as they become visible. If you find any, put larger creatures into the holding tanks.
Remove plants as the water levels drops, placing them in the holding tank. Take the opportunity to re-pot or divide plants if necessary.
Place decaying plant material on the side of the pond, so any smaller creatures hidden away can return to the pond.
Scoop up the silt from the base. You can use this on your border, but retain a little to add back to the pond as it will help re-establish tiny organisms.
Clean the pond liner with a scrubbing brush and water, bailing the dirty water out with a bucket.
Return the saved silt and any saved pond water. Next, refill with water (rain water if possible), positioning pond plants as you go.
Finish by returning the fish and any other creatures.
After cleaning out the pond, it can take several months or years to return to a balanced ecosystem.