Fish Diseases Decoded: How to Treat Ich, Fin Rot, Velvet, and Dropsy

Quick answer

If you spot white spots, frayed fins, or sudden bloating in your fish, you must act immediately to prevent the disease from spreading throughout your aquarium. Isolate the affected fish in a dedicated quarantine tank, identify the specific symptoms of Ich, fin rot, velvet, or dropsy, and begin targeted treatment using aquarium salt, copper, or antibiotics while optimizing your water quality.

Spotting white spots, frayed fins, or bloating in your aquarium? Learn how to vi

:::key-facts

• Ich (White Spot) looks like grains of salt sprinkled on the fish's skin and fins.
• Fin Rot causes ragged, frayed, or black-edged fins, usually triggered by poor water quality.
• Velvet presents as a fine, dusty gold or rust-colored sheen visible under a flashlight.
• Dropsy is a symptom of internal organ failure, characterized by severe bloating and protruding, pinecone-like scales.
• Quarantine is always the safest first step to prevent pathogens from spreading to healthy tankmates.
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Why it matters

An aquarium is a closed aquatic ecosystem. Unlike wild fish, which can swim away from localized pathogens, poor water quality, or aggressive tankmates, aquarium fish are entirely captive to their environment. When a pathogen enters a display tank, it can rapidly multiply and infect every inhabitant.

Stress is the ultimate trigger for almost all fish diseases. When fish experience stress—whether from ammonia spikes, sudden temperature fluctuations, improper pH, or bullying—their immune systems suppress. This allows opportunistic parasites, bacteria, and fungi to take hold. Understanding how to recognize these diseases early and knowing how to treat them safely is the difference between a minor setback and a total tank wipeout.

Ich presents as tiny, raised white spots resembling grains of salt sprinkled across the fish's body and fins.

What good looks like

A healthy fish is active, alert, and displays vibrant coloration. Their eyes should be crystal clear, and their scales should lie completely flat against their body, creating a smooth, uniform surface. Fins should be intact, fully extended, and free of tears, splits, or discolored edges.

Healthy fish swim effortlessly without scraping themselves against decorations (a behavior known as "flashing") or gasping for air at the water's surface. Their breathing should be steady and relaxed, with their operculum (gill cover) moving rhythmically. A robust appetite and curiosity toward their environment are the best indicators of excellent health.

Step-by-step

Treating fish diseases requires a systematic approach. Administering medications directly to your main display tank can disrupt your biological filter, kill live plants, and harm sensitive invertebrates like snails and shrimp. Follow this step-by-step protocol to treat your fish safely.

Phase 1: Set Up a Quarantine Tank (Hospital Tank)

Before administering any treatment, set up a separate, bare-bottom quarantine tank. This prevents medications from damaging your main display tank's ecosystem.

1. Use a clean, dedicated tank: A 5 to 10-gallon tank is usually sufficient for most community fish.
2. Install basic equipment: Add a heater to maintain a stable temperature and a simple sponge filter. Do not use chemical media like activated carbon, as it will absorb and neutralize medications.
3. Provide hiding spaces: Place clean PVC pipe joints or plastic plants in the tank to reduce stress. Do not use gravel or porous rocks, which can harbor parasites.
4. Fill with display tank water: Use water from your main aquarium to minimize acclimation shock for the sick fish.

:::pro-tip
Always keep a spare sponge filter running in your main tank's sump or filter compartment. This ensures you have a fully cycled, biologically active filter ready to go for your quarantine tank at a moment's notice.
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Phase 2: Treating Ich (White Spot Disease)

Ich is caused by the protozoan parasite Ichthyophthirius multifiliis. It has a multi-stage life cycle, and medications can only kill the parasite when it is in its free-swimming stage.

1. Raise the temperature slowly: Increase the water temperature in the quarantine tank to 82°F–86°F (28°C–30°C) over 24 hours. This accelerates the parasite's life cycle, forcing it into the vulnerable free-swimming stage faster.
2. Add aquarium salt or medication: Add 1 tablespoon of aquarium salt per 5 gallons of water to support the fish's slime coat and osmoregulation. Alternatively, treat with an over-the-counter medication containing malachite green or formalin.
3. Increase aeration: Warm water holds less oxygen. Add an air stone to ensure your fish can breathe easily during treatment.
4. Perform daily water changes: Change 25% to 50% of the water daily, vacuuming the bare bottom to remove fallen parasite cysts.

:::ask-boo
Is aquarium salt safe for scaleless fish like loaches and catfish?
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Phase 3: Treating Fin Rot

Fin rot is a bacterial infection (often caused by Pseudomonas or Aeromonas species) or a fungal infection that attacks the delicate tissue of the fins.

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Fin rot causes the delicate tissue of the fins to fray, split, and decay, often leaving inflamed red or black edges.
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1. Test and correct water quality: Fin rot is almost always caused by poor water conditions. Test for ammonia, nitrite, and nitrate. Perform a 50% water change immediately.
2. Administer antibacterial medication: If the rot is severe or has reached the base of the fin (the peduncle), treat the fish in quarantine with a broad-spectrum antibacterial medication like erythromycin or minocycline.
3. Maintain pristine water: Keep the water exceptionally clean during recovery. Pristine water is often enough to cure mild cases of fin rot without medication.

Phase 4: Treating Velvet (Gold Dust Disease)

Velvet is caused by Piscinoodinium, a tiny parasite that behaves similarly to Ich but is much smaller and highly contagious.

1. Black out the tank: The velvet parasite relies partly on photosynthesis to survive. Cover the quarantine tank completely with a blanket or dark plastic to block out all light.
2. Apply copper-based medication: Treat the water with copper sulfate, maintaining a therapeutic copper level (typically 0.15 to 0.20 mg/L) for 10 to 14 days. Use a copper test kit to monitor levels daily.
3. Provide heavy aeration: Copper can deplete oxygen levels and stress the fish's gills.

Phase 5: Treating Dropsy

Dropsy is not a specific disease, but rather a symptom of severe internal fluid accumulation caused by kidney failure or systemic bacterial infections.

:::warning
Dropsy is highly lethal and difficult to cure once scales begin to pinecone. You must act immediately. If the fish does not respond to treatment and is suffering, humane euthanasia using clove oil may be necessary.
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1. Isolate immediately: Move the fish to the quarantine tank to prevent other fish from consuming shed bacteria.
2. Add Epsom salt: Add 1 to 2 teaspoons of Epsom salt (magnesium sulfate) per 5 gallons of water. Epsom salt acts as a natural laxative and osmotic draw, helping to pull excess fluid out of the fish's body.
3. Dose broad-spectrum antibiotics: Treat the water or feed the fish food bound with a high-quality antibiotic like kanamycin or metronidazole to target the underlying bacterial infection.

:::ask-boo
How do I safely use clove oil to humanely euthanize a terminally ill fish?
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Signs something's wrong

To catch these diseases before they become fatal, monitor your aquarium daily for these specific visual and behavioral warning signs:

• Ich: Small, distinct, raised white spots resembling grains of salt on the body, fins, and gills. The fish may scrape its body against rocks, gravel, or decorations (flashing) to relieve itching.
• Fin Rot: Fins appear ragged, torn, or frayed. The edges of the fins may turn white, black, or red with inflammation. In advanced stages, the fin tissue completely erodes down to the body.
• Velvet: A fine, dusty, yellowish-gold or rust-colored coating across the skin. This can be difficult to see under standard aquarium lighting; use a flashlight in a dark room to spot the metallic sheen.

Velvet disease is best identified by shining a flashlight on the fish in a dark room to reveal a fine, gold or rust-colored dust.

• Dropsy: Severe abdominal bloating, lethargy, loss of appetite, and "pineconing," where the scales protrude outward from the body rather than lying flat.
• General Distress: Rapid gill movement, gasping at the water surface, clamped fins (holding fins tight against the body), or hovering listlessly in corners.

:::video{src="https://storage.googleapis.com/decennium-global.appspot.com/knowledge_assets/care_guides/fish-diseases-decoded-ich-fin-rot-velvet-dropsy/inline-4-1779990935404.mp4" poster="https://storage.googleapis.com/decennium-global.appspot.com/knowledge_assets/care_guides/fish-diseases-decoded-ich-fin-rot-velvet-dropsy/inline-4-still-1779990815529.png" alt="Top-down view of a betta fish with dropsy showing pineconing scales"}
Dropsy causes severe fluid retention, bloating the body and forcing the scales to protrude outward in a distinct pinecone pattern.
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When to call your vet

While many aquarists manage mild cases of Ich or fin rot at home, you should consult an aquatic veterinarian or an experienced fish health specialist if:

• Multiple fish die suddenly within a 24-to-48-hour period.
• Symptoms do not improve after completing a full course of over-the-counter medication.
• You suspect a highly contagious, systemic bacterial infection like Fish Tuberculosis (Mycobacterium), which can present with chronic wasting, spinal deformities, and open sores.
• You need precise diagnostic testing, such as a skin scrape or gill clip, to identify a resistant pathogen under a microscope.

Common mistakes

Avoid these frequent pitfalls to protect your fish during treatment:

• Leaving carbon in the filter: Activated carbon chemically extracts medications from the water. Always remove carbon inserts before dosing any treatments.
• Mixing medications blindly: Combining different medications without explicit instructions can create toxic chemical reactions or overload your fish's liver and kidneys.
• Stopping treatment too early: Many parasites, like Ich, have life stages immune to medication. You must complete the entire recommended treatment cycle, even if the fish looks completely cured.
• Changing water parameters too quickly: Rapidly changing water temperature, pH, or salinity to treat a disease can shock a weakened fish, causing fatal stress.
• Treating the display tank unnecessarily: Avoid dosing your main aquarium unless absolutely necessary. Medications can destroy your beneficial nitrifying bacteria, causing a catastrophic ammonia spike.

:::ask-boo
How long can the Ich parasite survive in an aquarium that has no fish in it?
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Quick FAQs

Can I use table salt instead of aquarium salt?

No. Table salt often contains additives like iodine and anti-caking agents (such as sodium ferrocyanide), which can be highly toxic to fish. Only use pure, non-iodized aquarium salt or rock salt.

Will copper medications kill my snails and shrimp?

Yes. Copper is extremely toxic to invertebrates, including snails, shrimp, and live corals. Never use copper-based medications in a tank containing these creatures, and be cautious when transferring treated fish back to their home tank.

How do I disinfect my equipment after a disease outbreak?

Soak nets, siphons, and containers in a solution of 1 part household bleach to 9 parts water for 10 to 15 minutes. Rinse them thoroughly with clean water, and then soak them in water treated with a heavy dose of dechlorinator (sodium thiosulfate) until the chlorine smell is completely gone.

Why did my fish get sick if I haven't added anything new?

Pathogens can exist in low, dormant numbers in your aquarium, or your fish may carry them subclinically. When water quality drops, or if the fish experiences stress from temperature swings or bullying, their immune system weakens, allowing the dormant pathogen to multiply and cause an outbreak.

Can fish diseases spread to humans?

Most fish pathogens cannot infect humans. However, Mycobacterium marinum (fish TB) can enter through open cuts on your hands, causing localized skin infections known as "fish tank granuloma." Always wash your hands thoroughly after working in your aquarium, and wear waterproof gloves if you have open wounds.