How to prevent belly rot in Medaka in winter

Als Medaka-eigenaar wil je natuurlijk het beste voor je vissen, ook tijdens de koude wintermaanden. Maar wist je dat voeren in de winter juist gevaarlijk kan zijn? Wanneer de watertemperatuur daalt, verandert het metabolisme van je rijstvissen drastisch. Hun spijsvertering vertraagt en ze hebben minder energie nodig om te overleven. Toch zie je je Medaka’s misschien nog steeds zwemmen en denk je: “Ze lijken honger te hebben!” Dit kan een fatale vergissing zijn.Buikrot, ook wel ‘dropsy’ genoemd, is een van de meest voorkomende winterproblemen bij vijver- en aquariumvissen. Deze aandoening ontstaat vaak door verkeerd voeren bij lage temperaturen. In dit artikel leer je precies wanneer je moet stoppen met voeren, hoe je de watertemperatuur correct meet en welke voedingsregels je moet volgen om je Medaka’s gezond door de winter te krijgen. Met drie simpele regels voorkom je onnodige ziektes en verlies van je geliefde rijstvissen.

What is belly rot and how does it develop?

Belly rot is a serious condition in which the belly of your Medaka swells from a build-up of fluid and undigested food. The fish gets a swollen, rounded belly and the scales often stand out like a pine cone. This happens because the internal organs, especially the kidneys and liver, no longer function properly. At low water temperatures the digestive system of your ricefish can no longer process food, so it stays in the intestines and starts to rot. The problem begins with anaerobic bacteria. These bacteria thrive in an oxygen-poor environment, such as the intestines of a fish with a slowed metabolism. When food stays in the digestive tract too long, you create perfect conditions for these harmful bacteria. They produce toxic substances that damage the intestinal wall and eventually affect the whole body of the fish. Once it reaches an advanced stage, belly rot is unfortunately often incurable and leads to the death of your Medaka.

Why are Medaka vulnerable in winter?

Medaka are naturally adapted to seasonal changes, but their bodies need time to adjust. In their natural habitat in Japan they eat almost nothing in winter. Their metabolism drops so sharply that they can go for weeks without food. In captivity many owners make the mistake of carrying on feeding because they think their fish are hungry. That is understandable, because Medaka often keep swimming actively, even at lower temperatures. The danger lies in the fact that the enzymes needed for digestion depend on temperature. Below 10 degrees Celsius these enzymes barely work anymore. Food you give then stays practically undigested in the intestines. This not only creates a breeding ground for bacteria, it also burdens the kidneys and liver, which have to process the waste. The result is a weakened immune system and a higher risk of infections and belly rot.

Check the temperature: when to stop feeding?

Water temperature is the most important indicator for your feeding schedule. So invest in a reliable pond or aquarium thermometer. Digital thermometers are the most accurate and give you a clear picture of the current temperature. Always measure at different times of the day, because the temperature can fluctuate, especially in outdoor ponds. The morning temperature is usually the lowest and gives you the best indication for your feeding decision. Follow this decision tree for feeding your Medaka:Is the water temperature below 10°C?

• JA → STOP FEEDING COMPLETELY
• NO → Go to the next question

Is the water temperature between 10°C and 15°C?

• JA → Feed a maximum of once a week, small amounts Medaka fish food
• NO → Go to the next question

Is the water temperature above 15°C?

• JA → Feed 2-3 times a week, adjusted portions Medaka fish food
• At 18°C and above you can gradually return to the normal feeding schedule

Practical measuring methods

Always measure the temperature before you feed, not afterwards. Place your thermometer in a representative spot in your pond or aquarium, not directly next to a heater or filter. In outdoor ponds it is wise to place the thermometer halfway down, because the bottom can be colder than the surface. Record the temperatures for a week to spot a pattern. Also watch for weather changes. A few sunny days can raise the water temperature temporarily, but that does not mean you can immediately return to normal feeding. The metabolism of your Medaka does not adjust that quickly. Wait until the temperature stays stably above the threshold for at least three days before you adjust your feeding schedule. This cautious approach prevents you from starting to feed too early and causing problems anyway.

Safe feeding between 10 and 15 degrees Celsius

The transition period between 10 and 15 degrees is the trickiest phase. Your Medaka still have some digestive capacity, but it is greatly reduced. In this period less is truly more. Feed only once a week and then give at most half of what a fish can eat in one minute. Choose easily digestible food such as special winter food or finely ground summer food. Observe your fish carefully after feeding. Healthy Medaka swim actively and have a flat or slightly rounded belly. If you notice the belly stays swollen after feeding or your fish become less active, you have given too much. Stop feeding immediately and wait until the temperature rises further. It is better to feed too little than too much, because Medaka can live for weeks on their fat reserves without harm.

The three golden rules in summary

1. Rule 1: always measure the temperature first
   • Use a reliable thermometer
   • Measure at different times of day
   • Wait for stable temperatures before you adjust the feeding schedule
2. Rule 2: follow the temperature thresholds strictly
   • Below 10°C: no food
   • 10-15°C: maximum once a week, small portions
   • Above 15°C: gradually build up to the normal schedule
3. Rule 3: observe and adjust
   • Watch the belly shape of your fish
   • Check for abnormal behaviour
   • When in doubt: don't feed

What to do at the first signs of belly rot?

If you still see signs of belly rot despite all precautions, act quickly. Isolate the affected fish if possible in a quarantine tank with clean, well-oxygenated water at the same temperature. Stop feeding immediately, including the other fish. Increase the oxygen supply in your pond or aquarium, because this helps the good bacteria and curbs the anaerobic bacteria. Special antibacterial treatments are available, but these only work at an early stage. Consult a specialised veterinarian or an experienced Medaka breeder for advice. Prevention is always better than cure, though. By sticking strictly to the temperature rules, you prevent belly rot from developing in most cases.

Continue reading

• Overwintering Medaka: tips for healthy fish

Frequently asked questions about winter feeding for Medaka

How long can Medaka go without food? At low temperatures (below 10°C) Medaka can easily go 2 to 3 months without food. Their metabolism drops so sharply that they barely use any energy. They then live off the fat reserves they built up in autumn. So don't worry if you don't feed for weeks during a severe frost.My Medaka swim actively at 8°C, aren't they hungry? No, activity does not automatically mean hunger. Medaka can still swim at low temperatures, but their digestive system no longer works well enough to process food. Feeding at this temperature does more harm than good, even if they seem interested in the food.Which food is best between 10 and 15 degrees? Choose special winter food with a high wheat germ content. This Medaka fish food is easier to digest at lower temperatures. You can also simply grind summer food into powder. Avoid high-protein food, because this is the hardest to digest.Can I heat my indoor pond so that I can keep feeding? Technically yes, but this is not advisable for Medaka. They need a natural resting period for their health and reproduction. Constant feeding and keeping a high temperature can lead to exhaustion and a shorter lifespan. Respect their natural cycle.How do I build up feeding in spring? Do this gradually. When the temperature stays stably above 15°C, start with small portions once a week. After two weeks you can increase this to twice a week. Only at 18°C and above do you return to the normal schedule of daily feeding. Pay close attention to your fish's reaction.What are the first symptoms of belly rot? The first signs are a slightly swollen belly, reduced activity and loss of balance. The fish may swim at an angle or struggle to get to the bottom. Later come protruding scales and bulging eyes. By these later symptoms the condition is often too advanced for successful treatment.

Temperature and feeding schedule overview

Water temperature	Feeding frequency	Amount	Type of food
Below 10°C	Do NOT feed	0	n/a
10-12°C	once every 10 days	Very small	Winter food
12-15°C	once a week	Small	Winter food / finely ground summer food
15-18°C	twice a week	Moderate	Mixed food
Above 18°C	3-5 times a week	Normal	Regular food

Preventing belly rot in winter is no rocket science, but it does take discipline and careful observation. By consistently measuring the water temperature and sticking strictly to the feeding rules, you give your Medaka the best chance of a healthy winter. Remember that these beautiful ricefish can go for months without food in nature, and that this is completely normal for them. You do them no favour by feeding when their body is not ready for it. The three rules are simple: measure the temperature, follow the thresholds and observe your fish. When in doubt about whether it is warm enough to feed, choose the safe option and wait a little longer. It is better to skip a feeding than to expose your Medaka to the risk of belly rot. With this knowledge and a little patience, your ricefish will come through winter healthy, and in spring you can enjoy their vivid colours and playful behaviour again. Good luck with your Medaka this winter season! Order our specially developed Medaka fish food