Getting your pond ready for autumn costs you an afternoon or three in October 2026, and they are the best spent afternoons of the year. The leaves are really falling now. In a 200 litre tub there is little buffer: whatever blows in during these weeks is still lying there in January, half rotted on the bottom.

Five steps keep the water clear. A net on top, the plants cut back, the sludge out, the feeding down and ammonia under watch. You need a landing net, a sharp pair of scissors and a bucket. For the medaka in your tub that is the difference between a winter they come through and one they just miss.

Getting your pond ready for autumn starts with the net

Every leaf that falls into the water now stays there. It sinks, discolours and breaks down, and that breakdown removes oxygen from the water that your fish need in the cold months. In a small pond or a tub with medaka you notice it sooner than in a large pool: there is less water to absorb the blow and at night photosynthesis stops.

What goes wrong can be predicted in three steps:

• Oxygen drops away, especially in the hours before sunrise, when plants consume oxygen themselves.
• Leaves sink to the bottom and form a sludge layer there that you will not get out again in spring.
• Breaking down releases ammonia, and in cold water that is converted only slowly.

The Japanese rice fish (Oryzias latipes) is tough and tolerates a cool winter better than most aquarium fish. Lack of oxygen is not a matter of species though, it is a matter of water. A fish that hangs quietly at the surface and gasps for air already has that problem.

String a net before the leaves fall

Getting your pond ready for autumn without a net is mopping with the tap running. Choose a fine-mesh net, 5 mm maximum for birch and oak, and 2 cm for coarser leaves. Pull it tight and a few centimetres above the water, not in it: a net that hangs in the water pushes the leaves down and does exactly what you wanted to prevent. Fix it with pins in the ground or with weights on the rim of the tub, because one autumn storm blows a loose net away just like that.

Check the net every week and after every storm. Wet leaves weigh a lot, sag through and tear the net. A torn net is worse than no net, because then you assume things are fine.

If you are late, daily skimming with a landing net is the emergency solution. Ten minutes a day in October saves you half a day of sludge vacuuming in November.

Cut back water lilies and marginal plants now

Water lilies, bulrushes and yellow water lilies are starting to die back now. Yellow leaves left lying are food for rot, so cut them off at the rhizome and take them out of the water. Do not put them on the bank, because the first rain shower washes them straight back.

Getting your pond ready for autumn is not a fight with your plants. Cut away about two thirds of the wilted parts and let the rest turn brown quietly. Green leaves still produce oxygen and give the fish cover, and a plant cut back to the root grows more slowly in spring. Plants in a basket you move a little deeper as soon as frost appears in the KNMI forecast ; the roots then stay below the frost line.

Vacuum the sludge while the water is still warm enough

Sludge is not mud from outside, it is what your fish and plants produce themselves: droppings, leftover food, dead plant parts. On the bottom of a tub there is easily a layer of a centimetre or two after a season. So you get your pond ready for autumn while the water stays above 10 °C: the bacteria still work then and the balance recovers after vacuuming.

This is how you go about it:

1. First fill a bucket with pond water, so you can rinse your hose and vacuum without bringing tap water into the tub.
2. Vacuum in strips from left to right and keep the end low, so you do not suck up a fish. Medaka are curious and like to swim in front of the tube.
3. Work the corners and the deepest spot extra well, that is where most of it lies.
4. Then add a bacterial preparation and test the water the next day.

For a 200 litre tub a siphon with a wide tube is enough. For a larger pond you hire a sludge vacuum or outsource it. More important than the equipment is that you replace no more than a third of the water, otherwise you lose the balance you wanted to protect.

Measuring ammonia keeps the winter peak out

Ammonia is the end product of everything that decays in your pond, and it is the substance that wrecks fish in November and December. At 20 °C bacteria convert ammonia into nitrite and nitrate within a few days. At 8 °C they take weeks over it, below 5 °C practically nothing. The supply does not stop when it gets cold, the processing does.

That is why measuring beats guessing:

A liquid test kit with colour cards is accurate enough. When in doubt measure twice, a day apart, and step in above 0.25 mg/l: change a third of the water, stop feeding, measure again the next day. For food the fish digest well, look in the Medaka shop. Undigested food is the biggest source of ammonia that you control yourself.

This is how you keep the water clear until spring

The order in which you do those steps matters. First the net, then the plants, then the sludge, and only after that measuring: every step lowers the organic load, so a measurement before you are done says little.

• Week 1, so now: string the net and skim leaves daily.
• Week 2: cut the plants back and take the cuttings away.
• Week 3: vacuum the sludge and top up the bacteria.
• After that: measure ammonia every two weeks until the water drops below 5 °C.

Getting your pond ready for autumn in October 2026 means nothing more than a rinse in March 2027. If you leave it, you start spring with a bottom full of rotting leaves, algae that profit from it and fish that spent the winter on their last reserves. That is the difference between maintenance and repair.

FAQ: frequently asked questions about the autumn round

The questions that come back most often in the mail, answered briefly.

When is it too late to step in?

As long as the water stays above 10 °C you can prune and vacuum. If it drops below that, getting your pond ready for autumn is limited to skimming leaves and measuring: you leave the bottom alone, because stirring up cold water costs more oxygen than the sludge costs.

Can a medaka get through the winter without a net?

In a tub with no trees above it yes, under an oak or birch no. Look at what lies on your terrace in October: if there are leaves there, they will be in your water in January too.

How often should I measure ammonia after the autumn round?

Every two weeks until the water drops below 5 °C, then once a month. After a thaw measure again, because bacterial activity briefly picks up and a peak can be released.

What do I do with the leaves I take out of the pond?

On the compost heap, not on the bank. A pile of leaves next to the pond blows back in at the first autumn storm, and then you have worked half a day for nothing.