Clear water, a view to the bottom of the pond, clearly visible fish, muscles and crustaceans, all surrounded by wonderful plants at the edge of the pond - this is what makes a water garden really fun. If only the old exasperating issue of algae didn't exist! If you look closely, you may suddenly no longer find it surprising to learn that those very garden beauties around the edge of the pond are, in fact, the death knell for water quality.
This becomes particularly clear in autumn: Leaves fall from the trees or are driven towards the water by the wind, and bush leaves and branches, once bent elegantly over the surface of the water, begin to die off and fall into the pond. All too often the autumn rains wash soil and mulch into garden ponds. If you don't do anything about it now, all kinds of garden waste will find its way - albeit naturally - into your water garden during autumn. With fatal consequences. Anything that decays in the water now is twice as dangerous. For one thing, the decay of garden waste in water produces fermentation gases which, once dissolved, can endanger fish if gas exchange from and within the water is not possible later on because the pond has iced over. The decayed remains of plants also release their nutrient loads into the pond water, a malicious inheritance indeed. These nutrients feed the algae in the coming year.
The most important rule of water gardens is therefore to prevent plants and nutrients from entering the water. To do so, spread a leaf net over ponds and streams to capture larger items. Trim back the plants at the edge of the pond so that no decaying leaves can fall into the water. Extendable shears are useful for this, since you can cut plants which are further away from the bank comfortably without getting your feet wet. Pond grippers with their long handles can also be used now to find and remove plants which have fallen into the pond. Hollow stems on bushes and grasses can be left on the plant unless there is a risk that they could fall into the water due to winds or snowfall. Garden insects like to spend the winter in them. Only cut these stalks in mid/late March and store them somewhere in your garden until the end of April so that the insects can leave them again. The remaining stalks still look beautiful throughout the autumn and winter when they are over-ripe or covered by snow - especially if they are illuminated during these dark seasons with effect lighting (garden lights).
Pumping out the sediment or duff from the pond with a mud pump has little effect at present. Fish and natural pond dwellers like to spend the winter holed up in the mud. In order to ensure the essential exchange of gases as mentioned above, you should keep the garden pond open in places if there is a heavy frost using an ice protection device. You should also ensure that sufficient bush, reed and grass stalks stick out through the ice. Beneath the sheet of ice they have contact with the water, which allows gas to be exchanged through them despite the ice sheet, like a straw.
You only have to remove fish from shallow ponds and allow them to overwinter in an aquarium if the pond might freeze over completely (for measurements, see below). Remove water lilies and other water plants at risk from ice from the garden pond now and place them in a bucket full of water so that they stay cold but frost-free. This also applies to water garden pumps if they are not too deep for the ice (which means 80 cm deep at our latitudes; in milder regions 60 cm is often fully adequate). The right time to remove the pumps and usually also the filtering system is when the water temperature is no longer regularly at 12 degrees. This is the critical temperature below which pond bacteria mostly stop operating and therefore do not remove excess nutrients from the water. However, if you operate a surface pump - a skimmer - you should leave the pump which operates it connected up until the risk of plant remains falling into the pond has finally passed in late autumn.