Common Aquarium Diseases & Treatment Lookup

When something looks wrong with your fish, you need fast identification. This is the lookup for common diseases by visible symptom — what it likely is, what causes it, and how to treat. Use this alongside the Disease Guide in Part 10 for deeper context.

At a glance: symptom to likely cause

The same symptom can point at several conditions, and a treatment for one does nothing for another — so identify before you dose. The detailed guides for each are linked below.

Identifying Disease: Where to Look

Symptoms appear in specific locations:

Skin/scales: white spots, slime, ulcers, color loss, raised scales Fins: rotted edges, holes, clamped tight, frayed Eyes: cloudy, popping out, swollen Belly: bloated, sunken, distended Mouth: white fuzz, fungal growth, broken Gills: rapid breathing, gasping, red Behavior: flashing, lethargy, hiding, gasping at surface

When in doubt, take a clear photo and search online + consult a knowledgeable forum.

Symptom-Based Disease Lookup

White Spots Like Salt Grains

Most likely: Ich (Ichthyophthirius multifiliis) — also called "white spot disease" Cause: Parasitic protozoa Severity: Highly contagious; can kill all fish quickly if untreated Visible features: Small white grains (like salt) scattered on body and fins; fish may flash against objects Treatment:

• Raise temperature to 82-86°F (speeds parasite lifecycle)
• Add aquarium salt (1 tbsp per 5 gallons, gradually) — for some species
• Use ich treatment (Ich-X, copper-based meds — careful with shrimp/scaleless fish)
• Treat for at least 2 full weeks (lifecycle dependent) Note: Treating mid-cycle when parasites are in free-swimming stage is most effective

Cottony White Patches on Body or Mouth

Most likely: Cotton mouth (Flexibacter/Columnaris) OR Fungal infection (Saprolegnia) Cause: Bacterial (columnaris) or fungal (saprolegnia) Severity: Columnaris is highly contagious and serious; saprolegnia often secondary to other issues Visible features: Fuzzy white growths; columnaris often around mouth; saprolegnia anywhere Treatment:

• For columnaris: antibacterial medication (Furan-2, Kanaplex)
• For fungal: antifungal medication (API Fungus Cure, methylene blue)
• Improve water quality, reduce stress
• May benefit from salt treatment (low salinity)

Frayed, Rotted, or Receding Fins

Most likely: Fin rot Cause: Bacterial — usually secondary to poor water quality or stress Severity: Slow progression; treatable if caught Visible features: Fins look ragged at edges; black or red discoloration where rotting; fins gradually shortening Treatment:

• Fix water quality first (water changes, test parameters)
• Treat with antibacterial medication (Furan-2, API Bettafix for mild cases)
• Improve diet; reduce stress
• Often resolves with just water quality fix in mild cases

Velvety Gold/Brown Dusting on Body

Most likely: Velvet (Oodinium/Piscinoodinium) Cause: Parasitic dinoflagellate Severity: Serious; spreads quickly Visible features: Dusty/velvety appearance, often gold or brown; fish may flash Treatment:

• Copper-based medication (be very careful with scaleless fish and invertebrates)
• Increase temperature carefully
• Reduce lighting (parasite is photosensitive)
• Treatment typically 10-14 days

Bulging or Cloudy Eyes

Most likely: Pop-eye (Exophthalmia) OR cloudy eye Cause: Bacterial infection, injury, water quality issues Severity: Pop-eye can indicate serious systemic issues Visible features: One or both eyes protruding from socket; cloudy white film over eye Treatment:

• Improve water quality
• Antibacterial medication (Kanaplex, Furan-2)
• Stress reduction
• May need quarantine

Swollen Belly, Pinecone-like Scales

Most likely: Dropsy Cause: Bacterial infection causing internal fluid retention Severity: Often fatal even with treatment Visible features: Bloated body; scales standing out from body like pinecone; lethargy Treatment:

• Kanaplex (antibacterial)
• Epsom salt baths
• Improve water quality dramatically
• Often a poor prognosis; consider humane euthanasia if severe

Lethargy + Hiding + Loss of Appetite

Most likely: Multiple possibilities — internal parasites, bacterial infection, water quality, age Severity: Variable Visible features: Not eating, staying in one spot, dark/pale colors, sometimes white stringy feces Treatment:

• Test water parameters first (often the cause)
• Check feeding routine
• Internal parasite treatment (Praziquantel or General Cure) if poo is stringy/white
• Quarantine if available

Gasping at Surface, Rapid Gill Movement

Most likely: Ammonia poisoning, low oxygen, or gill disease/parasites Cause: Multiple possibilities Severity: Emergency if many fish affected Visible features: Fish at surface gulping air; rapid gill movement; lethargy below Treatment:

• Immediate: large water change, increase aeration
• Test ammonia/nitrite/nitrate immediately
• If parameters fine, check for gill flukes (treat with Praziquantel)
• Check temperature (high temp = low oxygen)

Flashing (Rubbing Against Objects)

Most likely: Skin parasites (Ich, gill flukes, copepods, Chilodonella) Cause: Parasitic infections Severity: Variable; sign of underlying issue Visible features: Fish darts and rubs body on substrate, rocks, plants Treatment:

• Quarantine if possible
• Treat for likely parasites (start with Praziquantel for flukes)
• Examine closely for visible parasites
• Check water parameters

White Stringy or White Mucus Feces

Most likely: Internal parasites (Hexamita, Camallanus worms, etc.) Cause: Parasitic worms Severity: Slow but progressive Visible features: Long white stringy feces hanging from fish; sometimes loss of weight Treatment:

• Praziquantel-based treatment (PraziPro, API General Cure)
• Levamisole for camallanus worms
• Improve diet
• Treat for 2-4 weeks

Tumors, Bumps, or Cysts

Most likely: Tumors (sometimes cancer), bacterial cysts, or fish TB (Mycobacterium) Cause: Variable Severity: Often slow but progressive Visible features: Bumps under skin, sometimes growing over weeks Treatment:

• Often not treatable
• Mycobacterium can affect humans — handle with care
• Consider euthanasia if quality of life suffers
• Quarantine to prevent spread

Rapid Color Loss (Suddenly Pale)

Most likely: Stress or sickness, sometimes environmental Cause: Variable Severity: Sign of underlying issue Visible features: Once colorful fish now pale, washed out Treatment:

• Test water parameters
• Check for parasites or signs of illness
• Reduce stress (tankmates, lighting, water changes)
• Provide hiding cover

Black Patches/Spots (Body)

Most likely: Ammonia burns (during healing), or rare disease Cause: Usually environmental (recovering from ammonia damage) Severity: Healing process is good sign Visible features: Dark patches on body, often where skin was previously damaged Treatment:

• Keep water quality excellent
• These usually fade over weeks as fish heals
• If persistent or growing, investigate more

Treatment Quick Reference

Common Medications

API Furan-2 — Broad spectrum antibacterial; good for fin rot, columnaris, pop-eye API General Cure — Praziquantel + metronidazole; treats many parasites and protozoa Kanaplex (Seachem) — Antibacterial; effective for many bacterial infections Methylene Blue — Antifungal, antiparasitic; for hatching eggs, mild fungal infections Ich-X (Hikari) — Specifically for ich; safer than copper for most fish PraziPro — Specific praziquantel for parasites (gill flukes, internal worms) Levamisole — Specifically for Camallanus worms Salt — Aquarium salt (NaCl) for some bacterial/parasitic issues — careful with scaleless fish and plants

Treatment Best Practices

1. Quarantine if possible — separate sick fish; prevent spread.

2. Test water first — many "diseases" are actually water quality issues.

3. Use the right medication — antibacterial doesn't kill parasites; antifungal doesn't help bacterial infections.

4. Read instructions carefully — dosing for size of tank, duration of treatment.

5. Remove carbon from filter during medication — carbon adsorbs medication.

6. Don't mix medications without research — many combinations are dangerous.

7. Complete the full course — stopping treatment mid-way often causes recurrence.

8. Watch for sensitivities — scaleless fish, shrimp, snails react badly to many meds.

Medication Cautions

With shrimp:

• AVOID copper-based meds (kills them)
• AVOID most "tonic" salts
• Most ich medications are toxic

With scaleless fish (loaches, eels, some catfish):

• Use HALF dose of most medications
• Avoid copper-based meds
• Watch for stress signs

With snails:

• Avoid copper (deadly)
• Most other meds tolerable but check

Quarantine Procedure

When you have a sick fish:

1. Hospital tank — 5-10 gallon bare-bottom tank, sponge filter, heater, hiding spot

2. Move sick fish — net or cup transfer, minimize stress

3. Treat in hospital tank — main tank stays untreated unless ALL fish are affected

4. Daily water changes — 25% with appropriate temperature water

5. Watch for treatment effects — improvement, worsening, or no change

6. Complete treatment course — typically 1-2 weeks

7. Return fish to display — after treatment + observation period (3-7 days of normal behavior)

Prevention Beats Treatment

The best disease management is preventing disease:

Quarantine new fish — 2-4 weeks in hospital tank before adding to display Maintain water quality — most disease is opportunistic on stressed/weakened fish Avoid overcrowding — stress kills more fish than disease Quality food — varied, fresh, age-appropriate Regular observation — catch problems early Test water regularly — catch parameter issues before they cause disease Don't mix wild-caught and store-bought — different disease exposures

When to Euthanize

Hard but sometimes necessary. Signs:

• Disease too advanced for treatment
• Fish suffering visibly (gasping, lethargic, unable to feed for days)
• Quality of life severely impaired
• Risk of spread to other fish

Humane methods:

• Clove oil overdose (most accepted method)
• Quick decapitation (for skilled keepers)
• DO NOT flush live fish

Consult resources online for proper humane euthanasia procedures.

The Bottom Line

Disease identification starts with observation: where on the fish, what does it look like, what's the behavior? Common diseases by symptom — white spots (ich), cotton mouth/fuzz (columnaris or fungus), frayed fins (fin rot), velvety gold dust (velvet), bulging eyes (pop-eye), pinecone scales (dropsy), gasping (ammonia/oxygen issue), flashing (parasites), stringy feces (internal parasites). Match treatment to disease — antibacterial for bacterial, antifungal for fungal, antiparasitic for parasites. Always test water first; many "diseases" are actually water quality issues. Quarantine sick fish; remove carbon during treatment; complete full medication courses. Be careful with shrimp (no copper), scaleless fish (half doses), snails (no copper). Prevention beats treatment — quarantine new fish, maintain water quality, avoid overcrowding. Severe untreatable cases may require humane euthanasia. Most healthy fish stay healthy with stable parameters and stress-free environments.

Related guides

• The beginner's guide to fish disease
• Ich (white spot): identification and treatment
• Fin rot: causes, recognition and recovery
• Fungal infections: cotton-like growth
• Velvet, columnaris and other bacterial issues
• Internal parasites: signs and treatment
• Aquarium medications: when to use what
• Setting up and using a hospital tank

Got questions about your specific setup? Drop them in our Community Forum — we love helping new fishkeepers get started right.