Keep the water level in the container just below the level of the rim - check out the level after any Autumn rain or Winter snow
Regular maintenance in Autumn is particularly important for the smaller pond and patio container pond as you can accumulate significant amounts of rotting vegetation in a small volume of water.
Remove tree leaves that fall into the pond water.
Trim back the plant foliage so that tall upright plants will not drop their leaf into the water but leave them 4" above the water level.
Remove some of the rafting plant growth. Control excess spread and allow more light into the pond water in Winter.
Leave all oxygenating plants in place for now and thin out in Spring if you have more than 1/3rd of the volume filled with them then.
Stop adding extract of barleystraw once the clocks change in October but resume adding fortnightly again when the days brighten in Spring.
Doing these tasks will reduce the chances of formation of sludge
Manage any silt like this and never need to throw out the water and start afresh
Container ponds in Winter:
The plants and the water should be left in the pond during the Winter.
In freezing temperatures make sure the water level is below the slanting edge of the rim so there is room for the water to turn to ice. It will go up into the space under the rim temporarily rather than forcing itself outward against the walls of the containers
The surface of the container pond can freeze over without harm to the plants or damage to the container.
Avoid the whole container freezing solid.
If you are in a very cold area or anticipate a cold spell of weather wrap the container in fleece or bubblewrap for protection or drop a 'pop up' greenhouse over the whole mini pond.
Miniature waterlily will not thrive if they are trapped in frozen solid conditions. The rhizome is hardy when protected in water but not when trapped in ice.