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Fish Disease Identifier

Fish Diseases

Common Fish Diseases and Aquarium Health Problems

Browse common aquarium fish diseases, visible symptoms, possible causes, and what to check next. Start with water quality and breathing signs, then compare visible clues before relying on a photo-based match.

Water qualityBreathing signsVisible clues

Start here

Check water and breathing before waiting for a clearer photo match.

Gasping, collapse, pineconing, or several fish declining together can point to ammonia, nitrite, low oxygen, or temperature stress.

Goldfish in an aquarium, used as water-quality emergency context.

Priority guide

Ammonia Poisoning in Fish: Red Gills, Gasping, and Emergency Checks

Treat as urgent when fish are gasping, collapsing, or multiple fish are affected, especially if ammonia or nitrite tests are above safe levels.

Emergency checkRead guide ->

Image: AJ Gray / CC0 / resized and cropped for layout

Betta fish with velvet disease signs in an aquarium.

Priority guide

Velvet Disease in Fish: Dusty Coating, Fast Breathing, and Next Checks

Treat velvet-like signs with rapid breathing, surface gasping, or multiple fish declining as urgent because gill involvement can progress quickly.

Emergency checkRead guide ->

Image: Mydigitalife / CC0 / resized and cropped for layout

Neon tetra with dropsy-like swelling and raised scales.

Priority guide

Dropsy in Fish: Swollen Belly, Pineconing Scales, and Warning Signs

Get same-day expert help for pineconing, fast swelling, appetite loss, or lethargy. Severe breathing distress or collapse should be treated as urgent.

Same-day concernRead guide ->

Image: Citron / Public domain / resized and cropped for layout

Betta fish with infected wounds, used for bacterial lesion context.

Priority guide

Columnaris in Aquarium Fish: Cottony Patches, Mouth Changes, and Next Steps

Review the same day for fast-spreading patches, mouth involvement, gill signs, lethargy, or multiple affected fish.

Same-day concernRead guide ->

Image: Ryan O. Hershey / CC BY 4.0 / resized and cropped for layout

Compare common lookalikes

Fish disease clues that are easy to confuse

Disease
Hallmark clue
Often confused with
First check
Ich / white spot disease
Distinct salt-like white grains on fins, body, or gills.
Velvet, fungus, air bubbles, debris
Compare grain size and test water before choosing treatment.
Velvet disease
Fine gold or rust dusting plus fast breathing.
Ich, ammonia stress, gill flukes
Use angled light and treat breathing distress as urgent.
Columnaris
Gray-white patches, mouth erosion, saddle-like marks, or frayed fins.
Fungus, fin rot, injuries
Check spread speed, mouth/gill involvement, and water quality.
Ammonia poisoning
Gasping, red gills, lethargy, and multiple fish affected.
Gill flukes, velvet, low oxygen
Test ammonia and nitrite immediately.

Disease index

Browse fish diseases by likely pattern

Water quality emergencies

Goldfish in an aquarium, used as water-quality emergency context.

Image: AJ Gray / CC0 / resized and cropped for layout

Disease guide

Ammonia Poisoning in Fish: Red Gills, Gasping, and Emergency Checks

Ammonia exposure can burn gills and skin, causing gasping, red gills, lethargy, and sudden stress. Water testing is essential because photos cannot confirm ammonia.

Key signs

  • Gasping at surface, rapid breathing, or red and inflamed gills.
  • Lethargy, clamped fins, darting, or staying near filter flow.

First check: Test ammonia and nitrite before relying on photos.

Emergency checkRead guide

Parasites and spots

Aquarium fish with visible white spot disease signs.

Image: Thomas Kaczmarczyk / Djpalme / Public domain / resized and cropped for layout

Disease guide

Ich / White Spot Disease in Fish: Signs, Causes, and What to Do

Ich often shows as tiny white grains on the body, fins, or gills. Signs may match other irritation or parasite problems, so photo evidence and water checks both matter.

Key signs

  • Distinct salt-like white grains on fins, body, or near gills.
  • Flashing, rubbing, clamped fins, or sudden irritation.

First check: Compare salt-like grains with dust, fuzz, and bubbles.

Same-day concernRead guide
Betta fish with velvet disease signs in an aquarium.

Image: Mydigitalife / CC0 / resized and cropped for layout

Disease guide

Velvet Disease in Fish: Dusty Coating, Fast Breathing, and Next Checks

Velvet may look like a fine gold, rust, or dusty coating and can affect the gills. It can progress quickly, so breathing signs deserve attention.

Key signs

  • Fine gold, rust, tan, or dusty coating visible under angled light.
  • Rapid breathing, surface hanging, or staying near high flow.

First check: Use angled light and check breathing immediately.

Emergency checkRead guide
Corydoras catfish in an aquarium, used as gill and breathing context.

Image: Gabriel Resende Veiga / CC BY-SA 4.0 / resized and cropped for layout

Disease guide

Gill Flukes in Fish: Breathing Signs, Rubbing, and What to Check

Gill flukes are parasites that may irritate the gills and skin. Breathing changes, flashing, and excess mucus can overlap with other parasite or water-quality problems.

Key signs

  • Rapid breathing or one gill held partly closed.
  • Flashing, rubbing, mucus, or sudden dashing.

First check: Test ammonia/nitrite before assuming gill parasites.

Same-day concernRead guide
Fantail goldfish in a home aquarium, used as external parasite context.

Image: Ry362 / CC BY-SA 3.0 / resized and cropped for layout

Disease guide

Anchor Worm in Fish: Visible Threads, Sores, and Next Steps

Anchor worm may look like thin threads or small worms attached to the fish, often with redness or sores at the attachment point.

Key signs

  • Thin thread-like projection attached to skin, fins, or body.
  • Redness, swelling, ulcer, or sore at attachment point.

First check: Confirm a true attachment point and check for sores.

Same-day concernRead guide
Clown loaches with white spot parasite signs.

Image: ML5 / Public domain / resized and cropped for layout

Disease guide

Parasites in Aquarium Fish: Rubbing, Spots, Threads, and Next Steps

Parasites can be external or internal and may show through spots, flashing, mucus, breathing changes, weight loss, or visible attachments.

Key signs

  • Flashing, rubbing, mucus, white spots, dusty coating, or visible threads.
  • Rapid breathing or gasping when gills are irritated.

First check: Separate parasite signs from water irritation.

Same-day concernRead guide

Bacterial and tissue disease

Betta fish with infected wounds, used for bacterial lesion context.

Image: Ryan O. Hershey / CC BY 4.0 / resized and cropped for layout

Disease guide

Columnaris in Aquarium Fish: Cottony Patches, Mouth Changes, and Next Steps

Columnaris is a bacterial condition that may show as pale patches, mouth erosion, saddle-like marks, or fin damage. It can resemble fungus in photos.

Key signs

  • Gray-white patches on mouth, head, back, body, fins, or gills.
  • Mouth erosion, saddle-like back lesion, or cottony-looking but flat tissue.

First check: Check mouth involvement, spread speed, breathing, and water.

Same-day concernRead guide
Aquarium fish with visible skin irritation, used for growth and lesion comparison.

Image: Tze Sin, Tan / CC BY-SA 3.0 / resized and cropped for layout

Disease guide

Fungal Infection in Fish: Fuzzy White Growth and What to Check

Fungal growth often looks fuzzy, cotton-like, or thread-like on damaged tissue or eggs. It may follow injury, poor water quality, or another illness.

Key signs

  • Fuzzy, cottony, or thread-like growth attached to tissue.
  • Often starts on wounds, fin damage, eggs, or irritated skin.

First check: Find the wound or damaged tissue under the fuzz.

Same-day concernRead guide
Betta fish with infected wounds, used for bacterial infection context.

Image: Ryan O. Hershey / CC BY 4.0 / resized and cropped for layout

Disease guide

Bacterial Infection in Fish: Sores, Red Streaks, and Next Checks

Bacterial infections can show as sores, red streaks, fin damage, swelling, cloudy eyes, or lethargy. These signs overlap with injury and poor water quality.

Key signs

  • Red streaks, sores, ulcers, swelling, or fin deterioration.
  • Lethargy, clamped fins, appetite loss, or hiding.

First check: Find wounds and test water before considering treatment.

Same-day concernRead guide

Eye and mouth problems

Freshwater angelfish in an aquarium, used as eye and head context.

Image: Rjcastillo / CC BY 4.0 / resized and cropped for layout

Disease guide

Cloudy Eye in Fish: Causes, Checks, and Next Steps

Cloudy eye can be a symptom of injury, water stress, infection, or irritation. One cloudy eye often suggests trauma, while both eyes can point toward system-wide stress.

Key signs

  • Milky, hazy, scratched, or filmed eye surface.
  • One or both eyes may be affected.

First check: Check one eye vs both eyes and test water.

Monitor closelyRead guide
Discus fish in an aquarium, used as eye and head context.

Image: Bernard Spragg. NZ / CC0 / resized and cropped for layout

Disease guide

Popeye in Aquarium Fish: Swollen Eyes, Causes, and Next Checks

Popeye describes one or both eyes protruding. It can follow injury, infection, gas supersaturation, or water-quality stress.

Key signs

  • One or both eyes appear enlarged or protruding.
  • Cloudiness, redness, or swelling may surround the eye.

First check: Check whether one eye or both eyes are swollen.

Same-day concernRead guide
Betta fish portrait, used as mouth and face context.

Image: Daniella Vereeken / CC BY 2.0 / resized and cropped for layout

Disease guide

Mouth Rot in Fish: White Mouth, Erosion, and What to Check

Mouth rot describes white, gray, swollen, or eroding mouth tissue. It may be bacterial and can overlap with columnaris signs.

Key signs

  • White, gray, fuzzy, swollen, or eroded mouth tissue.
  • Spitting food, refusing food, or difficulty feeding.

First check: Check eating ability and compare with columnaris.

Same-day concernRead guide

Fins and tissue damage

Guppy in a home aquarium, used as context for fin health guides.

Image: Andrew Sheedy / CC BY-SA 4.0 / resized and cropped for layout

Disease guide

Fin Rot in Aquarium Fish: Signs, Causes, and Next Steps

Fin rot usually appears as frayed, shrinking, ragged, or discolored fin edges. It is often linked to stress, injury, poor water quality, or bacterial problems.

Key signs

  • Ragged, uneven, shrinking, pale, red, or dark fin edges.
  • Fin clamping, hiding, lower appetite, or reduced activity.

First check: Check water, nipping, and whether fin loss is spreading.

Monitor closelyRead guide

Swelling and systemic illness

Neon tetra with dropsy-like swelling and raised scales.

Image: Citron / Public domain / resized and cropped for layout

Disease guide

Dropsy in Fish: Swollen Belly, Pineconing Scales, and Warning Signs

Dropsy describes swelling and fluid imbalance rather than one single disease. Pineconing scales, severe bloating, and lethargy can be serious warning signs.

Key signs

  • Raised scales that look pinecone-like from above.
  • Sudden or severe body or belly swelling.

First check: Look from above for pineconing scales.

Same-day concernRead guide
Discus fish in an aquarium, used as head-area context.

Image: Bernard Spragg. NZ / CC0 / resized and cropped for layout

Disease guide

Hole-in-the-Head Disease in Fish: Pits, Erosion, and Next Steps

Hole-in-the-head is often discussed when fish develop pits or erosion around the head and lateral line. It may involve multiple stressors rather than one simple cause.

Key signs

  • Pits or erosions around the head, face, or lateral line.
  • Loss of condition, faded color, or reduced appetite.

First check: Review water, diet, stress, and lesion progression.

Monitor closelyRead guide

Swimming and buoyancy

Goldfish floating sideways with swim bladder disease signs.

Image: Humanfeather / Michelle Jo / CC BY 3.0 / resized and cropped for layout

Disease guide

Fish Floating Sideways: Swim Bladder Disease or Something Else?

Swim bladder problems can cause floating, sinking, rolling, or difficulty staying level. The visible behavior is a clue, not certainty.

Key signs

  • Floating, sinking, rolling, nose-up, tail-up, or sideways posture.
  • Excessive fin effort to stay level.

First check: Check water, feeding history, swelling, and posture.

Monitor closelyRead guide

Other aquarium health problems

Fish Disease Identifier

Still not sure what your fish has?

Scan a photo in Fish Disease Identifier and get likely disease matches, possible causes, and next steps to consider.

Educational only. Not veterinary advice.

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