The water in your tub is cooling fast these days, and with it the last window in which you can still work properly at the bottom closes. Removing sludge before winter is not a tidy-up job for a nice Saturday, it is the difference between a bottom that rots for months and a bottom that rests. In a 150 litre tub every thin layer weighs heavier than in a large pond, because there is little water to absorb mistakes.
Why removing sludge before winter tolerates no delay
Sludge is the sum of everything that has settled down since spring. Leaves from the trees around it, spent water plants, fish droppings, food that fell between the pellets and a thin layer of dead plant tissue that slowly falls apart. You see little of it while the water is clear, but it is there and it grows a little every week.
From early November more leaves join it, you may feed once a day at most, and the bacteria in the water work more slowly than in August. What you leave lying in the autumn of 2026 lies in the same spot in the spring of 2027. Only by then it is no longer leaves but a black layer you cannot get hold of with a net.
- Leaves and stems that sink to the bottom after the first night frost.
- Undigested food, especially pellets that stay caught between the plants.
- Fish droppings and the moulted skins of water fleas and snails.
- Dead leaves from oxygenating plants that come loose in autumn.
What sludge under a closed sheet of ice does to your water
Once a dense sheet of ice forms, the exchange with the air stops. No more oxygen diffuses in, while the bacteria on the bottom simply carry on breaking things down. In the bottom layer of a still tub the oxygen level then drops to nearly 0 mg per litre within a few weeks. That is exactly where your fish stand in the cold, because they head for the depth.
What remains is anaerobic rot. Other bacteria take over the work and produce methane, ammonia and hydrogen sulphide. The pH drops away, ammonia rises and the water turns a muddy grey. A thick layer is fuel for that process, and removing sludge before winter takes that stock away. Above the ice you see none of it: the water can look clear while the bottom has been running a process with no way back for weeks.
The Japanese rice fish occurs naturally in shallow, still water: rice fields, ditches and small pools where the bottom warms up in summer and turns cold in winter. It tolerates that swing well, as long as the water stays open and there is oxygen nearby (Wikipedia, Seriously Fish). A dense layer of sludge takes away exactly that.
Hydrogen sulphide: the gas you cannot smell under the ice
Hydrogen sulphide smells of rotten eggs. That is the only advantage it gives you: as long as the water is open you smell it and you can step in. Under a sheet of ice the gas stays trapped in the water and the concentration climbs steadily, with nothing to smell. Even at very low concentrations it attacks the gills and disrupts the uptake of oxygen into the blood.
Your fish then suffocate while there is still a fair amount of oxygen in the water. With medaka you rarely see it coming, because in cold water they move little anyway and often hang low in the tub. Watch for these signs when the ice has just gone or when you have kept a hole open:
- Fish hanging still just under the surface and gulping air.
- Gills that look dull or brown instead of bright red.
- Reacting slowly to food, or not reacting at all any more.
- Fish lying on their side without any visible injury.
How to carry out the last bottom service step by step
You do not need an expensive pump to remove sludge properly before winter. A siphon hose, a bucket and half an hour of calm work are enough for most tubs and tanks. Work on a dry, mild day and keep an eye on the frost forecast from KNMI in mind: the first night frost usually arrives in the Netherlands well before mid November, and after that the bottom is out of reach.
- Measure the water temperature first. If the water is above 8 °C you can work calmly; below 5 °C you postpone the job to a mild afternoon.
- Take leaves and plant debris off the surface with a net before you start on the bottom.
- Siphon the sludge off with a hose into a bucket. Let the bucket stand for 24 hours, pour the clear water back and put the sediment on the compost.
- Work in sections. Do not suck the whole bottom empty, because you need the biofilm on the walls and the gravel for water quality.
- Top up with water at roughly the same temperature so you do not give the fish a cold shock.
- Then test ammonia, nitrite and pH, and change another quarter of the water if any value sits above the safe limit.
Leave your filter alone. A sponge filter cleaned under the tap in November throws away bacteria you badly need in spring. At most rinse it in a bucket of tub water. Then put a net over the tub, because one rainy week with plenty of wind otherwise fills the bottom with leaves again.
| Method | How well it works | Effort | Suits |
|---|---|---|---|
| Siphon hose with bucket | Good, draws fine sludge with it too | Calm, half an hour per tub | Tanks and tubs up to 300 litres |
| Pond vacuum with suction head | Very good on coarse debris | More fuss, needs power | Larger tubs and small ponds |
| Net with fine mesh | Moderate, only takes leaves | Almost no effort | Emergency measure just before the frost |
| Full water change | Thorough, but hard on the fish | A lot | A tub you are emptying anyway |
| Sludge breaker with bacteria | Supporting at most | Almost no effort | Preventive in autumn, not a rescue |
Feed less in the coming weeks. Below 10 °C medaka need little, and what is not eaten sinks to the bottom where it tops up the layer you have just removed. A small portion twice a week is plenty in November, and once it freezes you stop altogether.
A clean tub goes into winter a good deal calmer
The sum is simple: an afternoon of work in November against a spring spent scooping dead fish out of the water. With a clean bottom, a hole in the ice and a filter that stays intact, you keep the oxygen level in your tub high enough to get through the darkest weeks. That is no guarantee, but it is the part you control yourself.
If you doubt your tub will make it, for instance because it is shallow or sits in a corner where the ice lingers, overwintering indoors is the safer choice. An unheated shed or garage where it does not freeze, with a container of water and a sponge filter, is often enough for a handful of medaka. You still remove the sludge before winter, before the container goes inside.
Verder lezen
- Algen in de medaka vijver: wat 16 jaar onderzoek leert
- Medaka verzorgen door de seizoenen heen
- DIY afzetmoppen voor medaka: 4 materialen getest
FAQ: removing sludge before winter in a small tub
How often should I clean the bottom of a medaka tub?
Thoroughly twice a year, in spring and just before winter. Of those two, removing sludge before winter is the more important. In between you net out what lies visibly on the bottom, especially after a week of strong wind or falling leaves.
Can I still remove sludge once there is ice on the tub?
No, better leave that. You break the ice up, cause pressure waves and chase the fish around in water in which they should be using as little energy as possible. Wait for a mild day in February or March.
Why do I smell nothing when there is hydrogen sulphide in the water?
Because under a dense sheet of ice the gas stays in the water and does not reach the air. The smell only gets to you when the ice melts or when you open a hole. If you do smell it then, assume the bottom has been without oxygen for too long.
Does an aerator or ice-free keeper really help?
It keeps a spot open where gas can escape and where some oxygen gets in. It does not dissolve a thick sludge layer, but it does give your fish air in the weeks when you cannot get to them.
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