Disclaimer: Hobbyist synthesis from labels, books, and experienced keepers — not veterinary diagnosis. Test water first. Verify doses on current product labels. Weigh salt (tsp last resort). Species differ. Escalate to an aquatic vet when needed. Full text: About / Disclaimer.

First response · Symptom index · Disease cards · Treatments & salinity · Escalate / stop · Gaps

Sick Fish Guide

Symptom → likely causes → what to check → treatments. Water quality before bottles.

1. First response / quarantine / water tests

Stop and look — How many fish? How fast did it start? Any new fish, plants, décor, meds, or water-source change in the last 2 weeks?
Test the water now — ammonia, nitrite, nitrate, pH, temperature. If you can: GH/KH. Write numbers down.
Aerate — Gasping or lethargy → add air stones / increase surface agitation immediately. Never turn off pumps overnight in planted/algae-heavy systems.
Match replacement water — Same temperature (±1–2°F), dechlorinated, pH close to tank. Sudden pH or temp swings cause “shock” that looks like disease.
Partial water change — If ammonia or nitrite > 0, or fish are gasping/streaked: large change with matched water. Do not stack medications on fried gills (dark red / brown gills).
Quarantine when practical — Move the sick fish (or the healthy ones) to a bare hospital tub/tank with heater, air stone, and daily water changes. Hospital = observation + control, not automatic antibiotics.
Quick physical (wet hands, support over water):
- Gills: cherry red = healthier; dark red/brown = toxins or heavy parasites; pale pink = anemia/stress/med toxicity; “hamburger” / gloppy = parasites.
- Belly: firm vs mushy vs rock-hard; red vent; dumpiness near peduncle.
- Mouth: red roof / white threads → think columnaris / systemic bacteria.
- Slime: thick/mucousy often parasites or toxins; “dry” patchy often columnaris/velvet.
- Poop: chunky food-colored = OK; long thin white = stress or internal bacteria; clear bubbly = not eating.
Rule out non-disease killers first — gravel stuck in mouth (“rockitis”), chlorine, ammonia/nitrite, overnight oxygen crash, pesticide drift into ponds.
Do not raise heat or dose harsh meds until parasites are considered. Warmth can explode flukes, costia, ich, etc.
Wait ~24 hours after water quality is corrected and light salt (see treatments page) before stacking more drugs — unless the fish is crashing (severe pinecone, open ulcers, mass flashing + gasping).

Salinity reminder: Supportive “light salt” for many freshwater goldfish protocols is about 0.1% = 1 ppt = 1000 ppm ≈ 3.8 g / US gallon. That is not 0.1 ppm. Use a kitchen scale; tsp is last resort. Full table on treatments.

2. Symptom index

One symptom can have several causes — check water first.

Symptom	Likely causes (start here)	What to check	Go to
White spots like salt/sugar grains	Ich (white spot); breeding stars if only on gill covers / pec rays	Temp swings; water params; flashlight at night	Ich
Dusty / yellow velvety patches	Oodinium (velvet)	Flashlight near dorsal; slime scrape if available	Oodinium
Flashing / scratching	Flukes, costia, trichodina, ich, poor water	Persistent vs occasional; chin red dots; dead organics	Flukes, Costia, Trichodina
Clamped fins, lethargy, thick slime	Chilodonella, water quality, early parasites	pH, ammonia/nitrite; cooler water history	Chilodonella
Gasping / piping at surface	Low O₂, gill flukes, ammonia/nitrite, acidosis, branchiomycosis	Aeration, morning pond O₂, gill look, water tests	Gill flukes, Acidosis
Pinecone scales / bloating / both eyes bulging	Dropsy (symptom, not a disease) — bacterial, parasitic, toxic, temp shock, kidney	Ascites vs pinecone; parasites first; NO heavy NaCl	Dropsy
White threads at mouth / cottony sores / red vent	Columnaris (often with organics + parasites)	Mouth, vent, “dry” slime, mushy belly	Columnaris
Red sores / ulcers	Aeromonas (white edge, red center) vs columnaris (red edge, white center); costia red dots under chin	Temp history; parasites; wound care	Aeromonas, Wound protocol
Red streaks in fins/body	Ammonia/nitrite burn vs systemic bacteria	Gill color; water tests before antibiotics	First response
Visible worm / Y-fork / shield with eyes	Anchor worm (Lernaea), fish louse (Argulus), leech	Naked eye; treat wound after removal	Anchor worm, Lice
Black spots on skin	Ammonia burn healing vs digenean black-spot (snails)	Water params; snails; 48h salt/WC trial for burns	Black spot
Floating / upside-down / sinking	Overfeeding, gulped air from dry floating pellets, constipation, eggs, toxins (H₂S), adhesions, rare true swim-bladder damage	Feed history; gravel siphoning; belly physical — not folk “garlic” cures	Floating / SBD
Yawning / head-standing suddenly	Gravel in mouth (rockitis)	Look in mouth; remove stone	Rockitis
Thin body, big head, eating but wasting	“Skinny disease” (bacterial), parasites, poor nutrition, mycobacteria (chronic)	Chronic vs acute; quarantine history	Skinny, Mycobacteria
Hole-in-head / lateral-line pits	Hexamita / nutrition / stress (more common in some tropicals than goldfish)	Diet, crowding, carbon media theory	Hexamita
Chronic lethargy + poor appetite, no parasites on scrape	Internal bacterial; mycobacteriosis; water/organ issues	History of failed multi-med courses; escalate	Mycobacteria, Escalate
Sudden death of largest fish first	Oxygen crash, toxins, pH/ammonia spike	Morning pond O₂; organics; aeration	First response

Parasite visual ID: diseases → parasite chart and parasitechart.jpg.

3. When to escalate / stop treatment

Stop or pause treatment if

Gills are fried (dark red/brown) and you were about to add formalin, malachite, PP, or other harsh baths — fix water and oxygen first; medicated food may be safer than water meds.
Fish worsens within hours of a new med or heat jump — reverse the last change (cooler matched water, fresh aerated water, neutralize PP with peroxide only if you know the procedure).
You mixed chemicals that must never combine (e.g. potassium permanganate + formalin → fire/explosion risk).
Course finished on schedule with no symptoms — stop; do not “keep dosing just in case.”
Only one eye bulges after bump/spawn — mechanical; clean water + light salt, not full dropsy heat protocol.

Escalate to an aquatic vet / lab when

Multiple fish die despite corrected water and a completed, appropriate course.
Chronic wasting, granuloma suspicion, or mycobacteria on the differential (zoonotic caution; gloves; no reliable OTC cure).
You need injectables, culture/sensitivity, or necropsy.
Legal / prescription drugs are indicated in your jurisdiction.
You are unsure of species-specific salt or heat tolerance (e.g. some Israeli koi lines, scaleless fish).

Do not

Stack every bottle on the shelf at once.
Raise temperature for “bacteria” while untreated parasites are likely.
Use heavy NaCl on pineconing dropsy (dropsy is generally treated as an electrolyte problem — prefer carefully dosed Epsom, not table salt).
Pop lumps/ulcers.
Flush euthanized fish (local disposal rules; clove-oil euthanasia notes are on the treatments page for humane end-of-life only).
Chase folk remedies (garlic floats, magic foods, etc.) instead of water tests and labeled protocols.

4. Gaps & honesty

Salt math: The hobby rule of thumb equates ~1 tsp/gal with 0.1%. Mass-based math says true 0.1% ≈ 3.8 g/gal (~0.75 tsp @ 5 g/tsp). This guide follows the corrected table. Weigh salt.
Products: Quick Cure, Romet-type foods, Dimilin, Baytril, Panalog, oxolinic acid, chloramphenicol, etc. may be unavailable, renamed, restricted, or vet-only. Prefer current labels + vet guidance.
Heat protocol (84–86°F): Goldfish-hobby approach for some bacterial/dropsy cases; risky if parasites present; oxygen demand rises; not universal for all species.
H₂O₂ fluke dip: A very short dilute dip is used; overdose destroys gills. Praziquantel per label is the other option.
Mycobacteriosis: No reliable OTC cure; zoonotic caution; isolate and escalate rather than stacking more meds.
Goldfish-centric: Tropical fish need different salt ceilings and temperatures.
Re-check: Verify against current labels and vet guidance before treating.