Medaka Diseases: Signs, Prevention, and What To Do First

1) White spot disease (白点病)

What it looks like: early on, medaka may “itch” and rub against surfaces. If it progresses, you’ll see small white dots on fins/body.
What usually triggers it: sudden water temperature changes. Japanese guides mention it often appears when seasons flip, after heavy rain cools outdoor tubs, or after a water change where temperature matching was skipped. (GEX)
What people do: start treatment quickly, keep temperature stable (not swinging), and use a medication that targets parasites as directed. Gentle aeration helps because stressed fish often struggle more when oxygen is limited.

2) Water mold / cottony growth (水カビ病 / ワタカムリ病)

What it looks like: white, cotton-like fuzz on the body or fins, often starting around an injury.
What usually triggers it: physical damage + conditions that let microbes bloom. Japanese sources repeatedly mention net abrasions, crowding-related scrapes, or fish bumping into objects. (GEX)
What people do: isolate, keep water clean, reduce stress, and use an appropriate medication for fungus-like conditions. Several guides advise pausing feeding during treatment to keep water quality from collapsing.

3) Fin rot (尾ぐされ病 / 尾腐れ病)

What it looks like: fins look thinner, clamped, frayed, or progressively “melt” away. Japanese sources mention it can be easy to miss from above, so side-view checks matter.
Main driver: water quality decline is repeatedly named as the #1 cause. Overfeeding and warm-season speed-ups can make deterioration happen faster than people expect. (GEX Medaka Disease Guide)
What people do: isolate the sick fish, improve water quality, and treat with an appropriate antibacterial-type medication as directed. One practical note from GEX: during medication baths, light aeration can help recovery; and because these bacteria can spread, the original container should be cleaned and the remaining fish monitored closely. (GEX)

4) Red spot disease (赤斑病)

What it looks like: red patches or streaks on the body/fins.
Common drivers: water quality issues + stress. Some Japanese guides note that in very mild early cases, improving water quality alone can help; but worsening cases may need medication. (Medaka-ya En)
What people do: immediate water improvement, reduce crowding, isolate if spreading, and consider an antibacterial-type treatment if symptoms progress.

5) Gill problems (エラ病) / “breathing hard” cases

What it looks like: rapid breathing, hanging at the surface, reduced activity. Sometimes gills look red or irritated.
Why it’s urgent: gills are where fish exchange oxygen, so even small damage becomes a big deal quickly.
What people do: isolate and increase aeration first. Then focus on water quality and consider whether the pattern looks parasitic (flashing, irritation) or bacterial (ongoing decline, fin/body damage). Some Japanese guides list salt bathing as supportive care in early stages and as an add-on alongside medication when appropriate. (Note (Black Rim Medaka), Medaka-ya En)

6) Dropsy / pinecone (松かさ病)

What it looks like: swelling/bloating plus scales lifting so the fish looks like a pinecone.
Reality check: across hobby literature, this is often a late-stage sign tied to internal infection or organ failure. You can try supportive care and antibacterial approaches, but outcomes are mixed.
What people do: isolate immediately, keep water pristine, use supportive care (gentle aeration; stable temperature), and consider an antibacterial medication as directed. Preventing it (quarantine, stable water, avoiding chronic stress) is usually more effective than “curing” it once advanced. (Mentioned as a key disease category in Medaka-ya En and Note.)

7) Wasting / “skinny disease” (痩せ細り病 / 痩せ病)

What it looks like: a fish eats but keeps getting thinner over 2–4 weeks, then becomes weak, wobbly, and dies.
What we actually know: A GEX lab report describes histological investigation where infections (including parasites and acid-fast bacteria in the genus Mycobacterium) were confirmed in kidney tissue of wasting medaka. The same report also notes that the cause has been unclear for a long time and effective countermeasures are not firmly established. (GEX Lab report)
What that means for keepers: treat this one as a tank management problem, not a quick “one medicine fixes all” issue. The safest approach is strict quarantine for new fish, early isolation at first signs of thinning, and preventing chronic stress/water deterioration that sets fish up to fail.

Other issues commonly listed in Japanese medaka disease guides

• Egg binding / egg retention (過抱卵 / 卵詰まり): females look swollen and uncomfortable, may become lethargic. Management usually focuses on reducing stress and improving conditions; severe cases can be fatal. (Medaka-ya En)

• Velvet (ウーディニウム病 / “pepper disease”): fine dusting appearance, irritation, clamped fins; treated as a parasite/protozoan-type issue. (Medaka-ya En)

• Prolapse (脱腸 / 脱肛): visible protrusion; often linked to stress, constipation, internal irritation. Supportive care and prevention are key. (Medaka-ya En)

• Pox / papilloma-like bumps (ポックス病 / 乳頭腫症): wart-like growths; often discussed as difficult to “cure,” with emphasis on maintaining strong baseline health and reducing stress. (Medaka-ya En, Note)

3) Quarantine is not optional if you care about long-term stability

If you only adopt one “serious keeper” habit, make it quarantine. Many outbreaks begin when a new fish (or even plants/water) brings in pathogens, or when new fish are stressed and become the spark that lights a problem already sitting in the system. This is especially relevant for wasting-type cases where prevention is much more reliable than treatment once advanced. (See GEX Lab report and disease-list guides like Note.)