Common Fish Diseases and Symptoms: Complete Identification Guide

Visual Symptoms:

• Salt-grain white spots on body, fins, gills
• Scratching/flashing against rocks/decorations
• Rapid gill movement (if gills infected)
• Lethargy + loss of appetite

Cause:

Protozoan parasite Ichthyophthirius multifiliis. Introduced via new fish, plants, or contaminated equipment.

Prevalence:

Most common fish disease — affects 60-70% of aquariums at some point.

Contagiousness:

⚠️ Highly contagious. Spreads to all fish in tank within 3-7 days.

Treatment:

• Medication: Ich-X, Malachite Green, API Super Ick Cure
• Dosage: 5ml Ich-X per 10 gallons, daily for 7-10 days
• Temperature: Raise to 86°F (speeds up life cycle)
• Duration: 7-10 days minimum (continue 3 days after spots gone)
• Cost: $12-$25 for 60-gallon treatment

Success Rate:

✅ 92-96% cure rate with medication + heat. Only 78-85% with heat alone.

Prevention:

• Quarantine all new fish for 4 weeks
• Avoid temperature drops >3°F in 24 hours
• Maintain low stress (good water quality, no overcrowding)

Visual Symptoms:

• Frayed fin edges (looks like torn paper)
• White/black discoloration on fin margins
• Progressive fin loss (disappears over days)
• Red streaks at fin base (advanced)

Severity Levels:

Cause:

Bacterial infection (Pseudomonas, Aeromonas) or fungal infection following fin damage. Triggered by poor water quality, fin nipping, rough handling.

Treatment:

• Medication: API Fin and Body Cure, Seachem KanaPlex, Maracyn
• Dosage: Follow package instructions (varies by product)
• Water Changes: 25% every 2 days during treatment
• Duration: 7-14 days
• Cost: $8-$18 for 40-gallon treatment

Success Rate:

✅ 85-98% depending on severity and water quality improvement.

Prevention:

• Keep ammonia and nitrite at 0 ppm
• Separate aggressive fish (fin nippers)
• Remove sharp decorations
• Feed high-quality, vitamin-rich foods

Visual Symptoms:

• Gold/rust dust coating on skin (looks like velvet fabric)
• Spots too small to count (vs. Ich’s countable spots)
• Scratching (more intense than Ich)
• Clamped fins + rapid gill movement
• Peeling skin (advanced stage)

How to Confirm:

Shine a flashlight at an angle on fish — velvet reflects gold/copper shimmer. Ich spots are white and raised.

Danger Level:

🔴 MORE DEADLY than Ich. Velvet reproduces 3-5× faster. Can kill fish in 3-5 days if gills are heavily infected.

Treatment:

• Medication: Copper sulfate (CuSO4), Cupramine, or Ich-X (contains copper)
• Dosage: Maintain 0.15-0.20 ppm copper (test daily with copper kit)
• Darkness: Cover tank with towels for 3 days (velvet needs light to photosynthesize)
• Temperature: Raise to 82-84°F
• Duration: 10-14 days
• Cost: $15-$30 for copper + test kit

Success Rate:

✅ 80-92% if caught early (within 48 hours). Only 40-60% if gills are severely damaged.

Prevention:

• Quarantine new fish 4 weeks (velvet has long incubation)
• Avoid sudden temperature changes
• UV sterilizer kills free-swimming velvet (50-70% reduction)

Visual Symptoms:

• Pinecone scales (scales raised outward due to fluid buildup)
• Swollen/bloated belly
• Bulging eyes (often)
• Pale gills
• Lethargy + sitting at bottom

Pinecone Test:

View fish from above. If scales stick out like a pinecone, it’s dropsy. If scales are flat, it’s constipation or bloating.

Cause:

NOT a disease itself — it’s a symptom of organ failure (usually kidney/liver) caused by bacteria (Aeromonas), poor water quality, or internal parasites.

Prognosis:

🔴 90-95% fatal. Once scales pinecone, organ damage is usually irreversible. Early intervention (before pineconing) has 30-40% success rate.

Treatment (Often Unsuccessful):

• Antibiotics: Maracyn Two, API General Cure (metronidazole)
• Epsom Salt: 1 tsp per 5 gallons (helps reduce swelling)
• Hospital Tank: Move to separate tank to monitor closely
• Duration: 7-14 days (if fish responds)
• Cost: $15-$40

Success Rate:

❌ 5-10% once scales pinecone. 30-40% if caught at early bloating stage before scales raise.

When to Euthanize:

• No improvement after 7 days of treatment
• Fish can’t swim or stay upright
• Refuses food for 5+ days
• Visible suffering (gasping, struggling)

Prevention:

• Maintain pristine water quality (0 ammonia, 0 nitrite, <20 ppm nitrate)
• Feed varied, high-quality diet
• Quarantine new fish
• Avoid overfeeding (causes internal bacterial growth)

Visual Symptoms:

• Floating at surface (can’t swim down)
• Sinking to bottom (can’t swim up)
• Swimming upside-down or sideways
• Struggles to maintain position
• Otherwise normal appetite/behavior

Cause:

Swim bladder malfunction due to:

• Constipation (70% of cases) — compressed swim bladder
• Overfeeding — bloated stomach presses on bladder
• Bacterial infection — inflammation of swim bladder
• Birth defect (fancy goldfish, balloon mollies) — genetic

Treatment (varies by cause):

For Constipation-Caused (Most Common):

• Fasting: Don’t feed for 2-3 days
• Pea Treatment: Feed blanched, peeled pea on Day 3 (acts as laxative)
• Epsom Salt: 1 tbsp per 5 gallons (softens stool)
• Duration: 3-7 days
• Cost: $3 (Epsom salt only)

For Bacterial Infection:

• Antibiotics: API General Cure, Maracyn
• Duration: 7-10 days
• Cost: $12-$18

Success Rate:

✅ 75-85% if constipation-related. Only 30-50% if bacterial or genetic.

Prevention:

• Soak dry flakes/pellets before feeding (prevents gas)
• Feed small portions 2× daily instead of 1 large meal
• Offer peas once weekly as fiber
• Avoid overfeeding (most common cause)

Visual Symptoms:

• Cotton-like white/gray tufts on body, fins, eyes
• Fuzzy appearance (like moldy bread)
• Usually on wounds or damaged areas
• May spread to healthy tissue if untreated

Fungus vs. Columnaris (Look-Alike):

Cause:

Fungus (Saprolegnia) grows on dead tissue, wounds, or weakened fish. Always a secondary infection following injury, stress, or other disease.

Treatment:

• Medication: API Fungus Cure, Malachite Green, Methylene Blue
• Salt Bath: 1 tbsp per gallon for 10-15 minutes (mild cases)
• Duration: 7-10 days
• Cost: $8-$15

Success Rate:

✅ 90-95% if caught early. Lower if underlying cause (injury, stress) isn’t addressed.

Prevention:

• Remove sharp decorations (prevent injuries)
• Maintain good water quality (fungus thrives in dirty water)
• Quarantine injured fish for observation
• Treat wounds immediately with antifungal

Visual Symptoms:

• White/gray patches on mouth, fins, gills
• “Saddleback” lesion across dorsal area
• Frayed fins (looks like fin rot)
• Rapid progression (grows in hours, not days)
• Lethargy + loss of appetite

Nickname:

“Mouth Fungus” or “Cotton Wool Disease” (though it’s bacterial, NOT fungal)

Danger Level:

🔴 Fast-acting killer. Can kill fish in 24-48 hours if gills or mouth are infected. Requires immediate treatment.

Treatment:

• Antibiotics: Maracyn, API Fin and Body Cure, Kanamycin
• Salt: 1 tbsp per gallon (slows bacterial growth)
• Temperature: LOWER to 75-76°F (Columnaris thrives in warm water 75-85°F)
• Duration: 10-14 days
• Cost: $15-$25

Success Rate:

✅ 70-85% if caught in first 24-48 hours. 30-50% if mouth/gills severely affected.

Prevention:

• Avoid high temps in summer (75-78°F max for vulnerable species)
• Don’t overcrowd (spreads rapidly in tight spaces)
• Quarantine new fish (common import disease)

Visual Symptoms:

• Small holes/pits on head and lateral line
• White, stringy feces
• Weight loss despite normal eating
• Color fading
• Lesions may enlarge over weeks

Commonly Affects:

Cichlids (especially Oscars, Discus, Angelfish), Gouramis

Cause:

Multifactorial:

• Hexamita parasite (intestinal flagellate)
• Nutritional deficiency (vitamin C, D, calcium)
• Poor water quality (high nitrate >40 ppm)
• Activated carbon use (removes trace minerals)

Treatment:

• Medication: Metronidazole (API General Cure), Praziquantel
• Nutrition: Feed vitamin-enriched foods (spirulina, bloodworms)
• Water Quality: Lower nitrate to <20 ppm via water changes
• Duration: 14-21 days (long treatment)
• Cost: $12-$20

Success Rate:

✅ 60-80% if caught early (small pits). 30-50% if large craters formed (indicates severe tissue loss).

Prevention:

• Feed high-quality cichlid pellets with vitamins
• Keep nitrate <20 ppm (weekly 30% water changes)
• Avoid prolonged activated carbon use (2 weeks max)
• Supplement with fresh vegetables (zucchini, peas)

Visual Symptoms:

• One or both eyes bulging
• Cloudy eye(s)
• Swelling behind eye
• Eye may rupture in severe cases

Unilateral vs. Bilateral:

Treatment:

• Antibiotics: Maracyn, API E.M. Erythromycin
• Epsom Salt: 1 tsp per 5 gallons (reduces swelling)
• Water Quality: Large water changes (50% immediately, then 25% daily)
• Duration: 7-14 days
• Cost: $10-$18

Success Rate:

✅ 70-85% for unilateral. 50-65% for bilateral (indicates more serious underlying issue).

Eye Recovery:

If eye ruptures, fish can survive but will be permanently blind in that eye. Fish adapt well to monocular vision.

Prevention:

• Maintain 0 ammonia, 0 nitrite (main water quality triggers)
• Remove sharp objects (prevent eye injuries)
• Treat any injuries immediately

Visual Symptoms:

• Thread-like worms sticking out of fish body (2-10mm long)
• Red, inflamed entry points
• Scratching
• Secondary infections at wound sites

What They Are:

NOT worms — they’re crustacean parasites (Lernaea). Females burrow into fish flesh with anchor-shaped heads.

Treatment:

• Manual Removal: Use tweezers to pull out visible adults (disinfect wound with iodine after)
• Medication: Dimilin, Dylox, Potassium Permanganate
• Repeat Treatment: Every 7 days for 3-4 weeks (kills newly hatched larvae)
• Cost: $15-$30

Success Rate:

✅ 85-95% with full treatment cycle. Reinfection common if treatment stopped early.

Prevention:

• Quarantine new fish and plants 4 weeks
• Avoid wild-caught fish/plants
• Don’t share nets between tanks

Visual Symptoms:

• Scratching/flashing (primary sign)
• Rapid gill movement
• Excess mucus production (cloudy slime)
• Clamped fins
• Lethargy

Types:

• Skin Flukes (Gyrodactylus): Live birth, spread rapidly
• Gill Flukes (Dactylogyrus): Lay eggs, cause breathing issues

Diagnosis:

⚠️ Invisible to naked eye. Diagnose by symptoms (scratching + no visible spots) + microscope examination of skin scrape/gill sample.

Treatment:

• Medication: Praziquantel, API General Cure, Prazi-Pro
• Salt Bath: 3 tbsp per gallon for 5-10 minutes (kills some flukes)
• Repeat Treatment: Day 1, 7, 14 (kills adults, then newly hatched larvae)
• Cost: $12-$25

Success Rate:

✅ 80-90% with full 3-dose treatment. Only 40-60% if treatment stopped after 1 dose.

Prevention:

• Quarantine all new fish 4 weeks
• Avoid overstocking (flukes spread via contact)
• Maintain excellent water quality (stress increases susceptibility)

Visual Symptoms (varies by type):

• Red streaks/patches (septicemia)
• Open sores/ulcers
• Cloudy eyes
• Frayed fins
• Bloating
• Lethargy

Common Bacterial Diseases:

Treatment:

• Broad-Spectrum Antibiotics: Maracyn Two, API Triple Sulfa, Kanamycin
• Hospital Tank: Isolate infected fish
• Water Quality: 50% water change immediately, then 25% daily
• Duration: 7-14 days
• Cost: $15-$30

Success Rate:

✅ 60-85% depending on severity and antibiotic resistance.

Prevention:

• Maintain 0 ammonia, 0 nitrite (bacteria thrive in poor water)
• Quarantine new fish
• Treat injuries immediately
• Avoid overstocking (stress weakens immune system)