Symptom-first quick reference
Fish Disease Decision Chart
Organize what you see, check urgent water and oxygen clues first, and compare the most relevant guides. This chart does not confirm a diagnosis.
- Version
- 1.0
- Reviewed
- July 2026
Start here
Water-first red flags override every branch
Follow the water-first path if any are present
- Surface gasping, severe labored breathing, collapse, or an unresponsive fish.
- Several fish becoming weak, breathing hard, or dying around the same time.
- A recent filter, pump, heater, power, water-change, chlorine, or chemical event.
- Rapid decline, pineconing with weakness, a deep wound, or fast tissue loss.
First checks
- 1.Restore or increase aeration and surface movement, and confirm the filter or pump is running safely.
- 2.Test ammonia and nitrite immediately; also record temperature and pH, and check for a chlorine or source-water problem after a water change.
- 3.Do not add another unverified chemical or mix medications while the cause is unclear.
- 4.Get urgent experienced help for collapse, severe breathing distress, sudden deaths, or rapid worsening.
If no red flag is present
Choose the clearest current sign
More than one branch may apply. Use the most urgent label and record what changed, when it started, and whether one or several fish are affected.
Breathing or gills
Start with how many fish are affected and whether the change was sudden.
If: Several fish gasp, breathe fast, or gather near flow
Act nowCompare: Low oxygen, ammonia, nitrite, heat, filter failure, or a recent water/chemical event.
Check first: Use the water-first path now. Do not wait for spots or a clearer photo.
If: One fish breathes fast, rubs, makes extra mucus, or holds one gill partly closed
Check todayCompare: Gill irritation, gill flukes, velvet, ich affecting the gills, or an individual injury.
Check first: Test water and oxygen first, then review new livestock and visible skin or gill signs.
If: Gills suddenly look red, inflamed, pale, or dark
Check todayCompare: Ammonia or nitrite irritation, low oxygen, chlorine, parasites, or chemical exposure.
Check first: Record whether one or many fish are affected and test ammonia and nitrite immediately.
White spots or coating
Compare the texture and size under neutral, angled light.
If: Separate white marks look like salt grains
Check todayCompare: Ich / white spot disease, especially with rubbing or clamped fins.
Check first: Check breathing, water, recent livestock, and whether spots are spreading across fish.
If: A fine gold, rust, or dusty sheen appears with hiding or fast breathing
Act nowCompare: Velvet-like irritation, which can involve the gills before the coating is obvious.
Check first: Treat breathing distress as the priority and use angled light only after water-first checks begin.
If: The mark is fluffy, cottony, or thread-like on a wound or fin
Check todayCompare: Fungal or water-mold growth on damaged tissue, with bacterial look-alikes.
Check first: Check the underlying wound, aggression, fin damage, spread speed, and water quality.
If: The patch is flat, mouth-focused, saddle-like, eroding, or spreading quickly
Check todayCompare: Columnaris-like bacterial disease or mouth rot rather than simple fungus.
Check first: Check mouth and gill function, spread speed, temperature, injury, and water quality.
Fins or tissue damage
Decide whether the edge is a stable tear or is actively changing.
If: There is one clean split after chasing, netting, sharp decor, or intake contact
MonitorCompare: Mechanical injury or fin nipping before assuming infection.
Check first: Inspect tank mates, decor, equipment, and old photos; still test water because it affects healing.
If: The edge becomes ragged, recedes, or develops pale, dark, or red discoloration
Check todayCompare: Fin rot, repeated nipping, bacterial irritation, or secondary tissue damage.
Check first: Compare dated photos and check water, aggression, edge color, and redness near the fin base.
If: Red streaks, ulcers, bloody areas, or deep tissue loss appear
Check todayCompare: Water irritation, injury, or a bacterial/systemic problem.
Check first: Test water, document spread, and get prompt help if the fish weakens, stops eating, or wounds deepen.
Belly, scales, or buoyancy
Use both a side view and a top view before choosing a pattern.
If: Scales stand out like a pinecone with whole-body swelling or weakness
Check todayCompare: A serious dropsy-like systemic pattern rather than ordinary belly fullness.
Check first: Check water, breathing, appetite, and spread; collapse or labored breathing returns to the act-now path.
If: The fish floats, sinks, rolls, or cannot stay level
Check todayCompare: Buoyancy trouble from digestion, injury, water stress, infection, or systemic illness.
Check first: Check water, feeding history, swelling, pineconing, strength, and breathing before naming a cause.
If: Only the belly is round, the scales lie flat, and the fish remains alert
MonitorCompare: Feeding-related fullness, eggs, normal body shape, parasites, or an early health problem.
Check first: Review recent feeding, waste, sex or egg status, water, and whether the shape changes or progresses.
Eyes or mouth
One eye and both eyes often point to different first checks.
If: One eye becomes cloudy after a collision, fight, netting, or decor contact
MonitorCompare: A local scratch or injury, while still checking water and progression.
Check first: Inspect hazards and aggression, test water, and photograph whether cloudiness or swelling changes.
If: Both eyes become cloudy, or several fish show eye or gill irritation
Check todayCompare: A tank-wide water, chemical, or environmental problem before an isolated injury.
Check first: Test ammonia and nitrite and review pH, recent water changes, chlorine, and tank-wide signs.
If: One or both eyes visibly protrude or swell
Check todayCompare: Popeye-like injury, infection, fluid imbalance, or water stress.
Check first: Note one eye versus both, check water and injury history, and watch feeding and vision.
If: The mouth is white, fuzzy, swollen, eroded, or unable to take food
Check todayCompare: Mouth rot, columnaris-like disease, fungus, or injury.
Check first: Check spread speed, breathing, feeding ability, water, and whether the patch is flat or fluffy.
Behavior without a clear visible sign
Behavior is useful context, but water and tank history come first.
If: The fish repeatedly rubs or flashes against objects
Check todayCompare: Parasites such as ich, velvet, or flukes, plus ammonia, nitrite, pH, chlorine, or debris irritation.
Check first: Test water and look for spots, dusting, mucus, breathing change, and recent livestock.
If: The fish ignores or spits food but has no obvious external mark
MonitorCompare: Water or temperature stress, bullying, mouth trouble, swelling, parasites, or internal illness.
Check first: Check water, mouth, belly, breathing, waste, tank mates, and how long appetite has changed.
If: Several fish suddenly hide, lie down, react slowly, or become weak
Act nowCompare: Oxygen, ammonia, nitrite, temperature, pH, chlorine, filter, or chemical stress.
Check first: Use the water-first path now, especially after a power, filter, heater, or water-change event.
Look-alike table
Compare the pattern, not one color word
| Visible pattern | Compare with | Useful clues | Check first |
|---|---|---|---|
| Separate salt-like grains | Ich more than dust, fuzz, or a flat lesion | Distinct attached grains; rubbing or clamped fins may occur. | Breathing, water, recent livestock, and spread across fish. |
| Fine dust versus cottony growth | Velvet-like sheen versus fungus/secondary growth | Dust is very fine and may pair with fast breathing; cotton has visible texture and often sits on damage. | Use angled light, but prioritize breathing and water first. |
| Cottony tuft versus flat mouth/saddle patch | Water mold/fungus versus columnaris-like disease | Fluffy strands on a wound differ from a flat, fast-spreading, mouth- or back-focused patch. | Location, texture, spread speed, mouth/gill function, and water. |
| Clean fin split versus receding edge | Mechanical injury/nipping versus fin rot or bacterial irritation | A stable clean split follows a hazard; progressive loss develops changing, discolored, or inflamed edges. | Old photos, tank mates, decor, filter intake, and water. |
| One cloudy eye versus both eyes | Local injury versus a wider water/environmental concern | One eye may follow trauma; both eyes or several affected fish shift attention to the tank. | Ammonia, nitrite, pH, chlorine event, injury, and protrusion. |
| Belly fullness versus pineconing | Feeding, eggs, body shape, or parasites versus a dropsy-like systemic pattern | Scales that stand away from the body are easiest to see from above. | Top view, appetite, waste, buoyancy, breathing, and progression. |
Blank record
Water and behavior log
Tank / fish: ______________________________
| Date / time | Ammonia | Nitrite | Nitrate | pH | Temperature | Behavior / notes |
|---|---|---|---|---|---|---|
| Row 1, Date / time | Row 1, Ammonia | Row 1, Nitrite | Row 1, Nitrate | Row 1, pH | Row 1, Temperature | Row 1, Behavior / notes |
| Row 2, Date / time | Row 2, Ammonia | Row 2, Nitrite | Row 2, Nitrate | Row 2, pH | Row 2, Temperature | Row 2, Behavior / notes |
| Row 3, Date / time | Row 3, Ammonia | Row 3, Nitrite | Row 3, Nitrate | Row 3, pH | Row 3, Temperature | Row 3, Behavior / notes |
| Row 4, Date / time | Row 4, Ammonia | Row 4, Nitrite | Row 4, Nitrate | Row 4, pH | Row 4, Temperature | Row 4, Behavior / notes |
| Row 5, Date / time | Row 5, Ammonia | Row 5, Nitrite | Row 5, Nitrate | Row 5, pH | Row 5, Temperature | Row 5, Behavior / notes |
Recent water/filter change: __________________
New fish or plants: __________________________
Feeding/medication event: ___________________