A wet dry vacuum cleaner can be very useful when your pond has accumulated too much rotting vegetation in the bottom to remain healthy for the fish and plants.
Tank - Have a tank or container with some pond water ready to receive the fish from your pond.
Cardboard - Have a large piece of cardboard ready to put the vacuum cleaner top on.
Drain point - Identify a drain point to empty the vacuum at.
Dead vegetation - Remove as much dead vegetation and rubbish from the pond as possible.
Water plants - Remove any water plants to a safe place where they will not be in direct sunshine or dry out. Lay out a hose to refill the pond.
Removing the Bulk of the Water
Although a wet and dry vacuum will empty the pond it could prove tiresome. Carrying the vacuum to empty it every few minutes will become hard work. To ease the job it might be possible to use the pond pump to drain much of the water. You would need to connect a hose to the pump outlet and then take the other end of the hose to a suitable drain. The pond water can be safely disposed of down a main drain.
Using the Wet Dry Vacuum
If the pump is draining some of the water you can concentrate on using the vacuum to remove the dirt from around the edge of the pond as the level falls. When the vacuum is full it will stop sucking up water and make a different noise. Remove the top of the vacuum and set it down on the cardboard.
If the vacuum has a removable bucket use that to take the water to the drain point otherwise you will have to carry the vacuum base. Continue in this way until the water level is low enough to safely catch the fish and move them into the container. When the pond pump is no longer fully submerged, switch it off and disconnect the hose.
Cleaning the Bottom
As you get to the bottom of the pond the vacuum will be sucking up more rubbish. If the filter under the top of the vacuum gets clogged up, you will have to clean it with the hose. As the water is drained away you will be able to lightly scrub the sides and bottom of he pond to loosen any algae and encrusted dirt.
Wash the sides down with the hose as you scrub them, and use the vacuum to drain it from the pond. The pond does not have to be spotlessly clean so you don’t need to try to remove all the marks.
Setting the Clean Pond Up Again
Once you have finished cleaning the pond you can start to fill it with the hose. Reconnect the pump. When the water is deep enough replace the plants and the fish. Since you are using tap water to refill the pond you should add some dechlorinator.
Although this was a long and dirty job, your pond will look better and will not need cleaning for a year or so.