How to Clean a Garden Pond
It is easier to clean a rigid sided pond than it is one with a flexible liner. With the latter there is always the risk of tearing the material as you walk on it or scrub it vigorously, particularly if the liner is ten years old or more. It can be cleaned, however, and as long as you are careful where you stand and what you do, the process you need to follow is the same as for any kind of pond.
Start by removing any pond plants in containers; these can be put to one side, in a shady part of the garden, and can remain there for two or three days if necessary. Larger plants should then be tackled. Often these will be growing rampantly in the mud at the bottom of the pond. Plants like irises, for example, can become so large that it is impossible for one person to move them. Use a knife to reduce such plants piece by piece, until they are moveable.
When any large amount of plant material is removed from a pond, always check the crown and roots for fish or frogs that have got caught up in it. Blanket weed may be a problem, and it is important to get as much out of the pond as possible, at any time of year.
Next, start to reduce the level of the water. You can bale it out with buckets, but this is a long and tiring job, and the water will end up being thrown around the garden indiscriminately. It is better to attach the end of a hose to a submersible pump (perhaps one that normally operates a waterfall or fountain); the other end of the hose can be directed to specific places in nearby borders.
After half of the water has been removed, stop to catch the livestock. Net out all fish, frogs, newts and other large creatures, (insects are best left to fend for themselves). This procedure can be long and drawn out, particularly as you will have stirred up the mud and cannot see the fish easily. However, as there is a much-reduced volume of water, the creatures will be more concentrated together. You will need a degree of patience and dexterity; this one part of the process will seem to take a disproportionately long time.
When you have succeeded in netting the fish, store them in a large, watertight container — such as a plastic dustbin. Do not put the amphibious creatures into the dustbin, unless there is a convenient route out for them —frogs and toads can drown if they are not able to breath air from time to time, and in a deep dustbin they can quickly exhaust themselves trying to stay afloat.
Once all of the livestock has been removed, you can turn the pump on again and empty the rest of the water.
There will be a thick sludge in the bottom of the pond. Do not allow this to be sucked up by the pump, as it could do irreparable damage to it. This silt will need to be removed by hand — an old dustpan is useful for scooping it out, rather than a spade or shovel, which might damage the pond. Once the sludge has gone you will be able to clean off the areas of liner that are stained with algae. You need not bother with areas below the waterline, as once the pond is refilled, you will not see them.
At this stage make any repairs to the liner. After you are satisfied that the liner is clean and sound, with no leaks, tears or holes in it, you can refill the pond with clean tapwater, using a hosepipe. Reposition the plants after a day or two, but allow the water to settle for a week or so before you reinstall the fish.