Ranchu diseases and health problems
Most ranchu problems are one of seven things, and five of them start with the water. Here they are in the order you are likely to meet them.
Before anything else: test the water
Ammonia and nitrite must be zero, nitrate under 20 ppm, pH stable between 7 and 8. If any number is off, fix it with water changes first. Half of what follows resolves on its own once the water is right, and no medication works in bad water.
1. Floating, sinking, tilting
Gut and swim-bladder trouble from floating food, overfeeding, cold water or nitrate. Fast two days, feed peas, switch to sinking food. Full write-up: why is my ranchu floating.
2. Wen infection
White cottony patches or a slimy film in the folds of the head growth. Bacteria or fungus in trapped debris. Clean water plus 0.1 to 0.3% aquarium salt clears mild cases in a week. Persistent cases need an antibacterial. Small white pimples on the wen that come and go are new growth, not infection.
3. Ich (white spot)
Salt-grain white spots on body and fins, flashing, clamped fins. A parasite that arrives with new fish. Raise temperature slowly to 78 °F, treat the whole tank with an ich medication or salt at 0.3%, and continue for a week after the last spot goes. Quarantine new fish to avoid it.
4. Flukes
Flashing, rubbing on objects, excess slime, gasping, no visible spots. Gill and skin flukes are the hidden cause of many "mystery" goldfish deaths. Treat with praziquantel; two courses a week apart.
5. Fin rot and red streaks
Ragged, milky fin edges, or red lines through the tail. Bacterial, almost always following poor water or a fluke problem. Water changes and salt first; an antibacterial if it spreads.
6. Ulcers
Open red sores, often on the body. Aeromonas or similar bacteria, usually on a stressed fish. Improve water, salt to 0.3%, and use a broad-spectrum antibacterial. Deep ulcers may need injectable antibiotics from a fish vet.
7. Dropsy
Swollen body with scales sticking out like a pinecone. Organ failure, usually kidney, from a long-standing infection or bad water. Prognosis is poor. Isolate the fish in shallow, warm, salted water and treat with an antibacterial; some recover if caught very early.