Some Signs of a Sick Fish


Apr 21, 2009
Some Signs of a Sick Fish

by Dr. Adrian Lawler
(retired) Aquarium Supervisor (l984-l998) J. L. Scott Aquarium Biloxi, Ms 39530
Original to Aquarticles

Signs of fish disease (or fish stress) can be any abnormal behavior or any change in fish observed externally. Internal signs of disease can be ascertained by dissection of a living anesthetized fish or a dead/sacrificed fish. Disease organisms or parasites can rapidly increase in numbers in a closed, recirculating small environment as a tank. One should closely observe the fish and tank in order to ascertain any possible problem before a valuable specimen, or a tank, is lost. Daily tank checks should be done to observe for problems. Following is a list of some of the signs one may observe (in freshwater to salt water tanks) indicating a possible problem, with notes, or some possible causes (all causes may not be listed), listed in parentheses (no parentheses indicates fish disease signs may be caused by a number of causes).

Tank Signs (conditions which could indicate present, or future problems with fish)
----Cloudy water (ammonia, bacteria, algae bloom, dinoflagellates [free-living or parasitic], air off, filter
off, decaying organics, hole in substrate over under gravel filter, suspended particulates)
----Protein foam on surface of water (fish, squid, etc. cut up for food not washed well, excess food in
tank, something organic decaying in tank)
----Hole exposed to under gravel filter (allows water to bypass substrate and decrease biological
----Missing or dead specimens, which could be decaying in tank and on the way to fouling tank or
causing a bacteria or algae bloom
----Uneaten or excess food in tank; dying plants; sheds from crustaceans (could foul tank, etc)
----Air off, filter or other support equipment not working correctly (could lead to decline in biological
filtration and disease control, and result in future tank problems)

Fish Behavior
----Violent reaction of thrashing, darting, and dying mid-gasp (pH shock or quickly lethal toxins)
----Shimmy (staying in one place and moving side to side) (Bacteria, missing salts)
----Equilibrium decreased, cannot maintain normal position in water (bacteria, dinoflagellates, toxins)
----Scratching on objects in tank (bacteria, parasites, toxins)
----Twitching (bacteria, parasites, toxins, skin irritant)
----Spastic, uncoordinated movements
----Sluggish, lethargic
----Swimming with head up, tail down, at surface
----Constant swimming at surface (dinoflagellates [free-living or parasitic], toxins, pH shock)
----Trying to jump out of tank (dinoflagellates [free-living or parasitic], pH shock, toxins)
----Sticking head out of water and squirting (black-flushing gills) water (dinoflagellates, gill problems)
----Normally bottom fish now at surface
----Normally mid-water or surface fish now at bottom
----Normally hiding fish now out and visible
----Lying on bottom and gasping (near death; heavy bacteria, parasite or dinoflagellate infection)
----Swimming to top, then sinking to bottom

Breathing, opercules, gills
----Gasping, irregular opercular beat, rapid opercular beat (bacteria, dinoflagellates, parasites, toxins)
----Squirting water (parasitic dinoflagellates, gill parasites, gill infection)
----Opercules flared (gill parasites, gill infection, toxin damage to gills, or large gill parasite, as isopod)
----Mouth not closed (usually indicates gill/oxygen problem of some sort, heavy gill infection/infestation;
something caught in buccal cavity or throat = food, substrate, etc)
----Rapid or irregular opercular beat (bacteria, dinoflagellates [free-living or parasitic], toxins,
hyperplasia, parasites )

----Not eating (bacteria, parasites, toxins, severe liver damage, internal injury or problem, cancer)
----Losing weight even though eating (Mycobacterium, toxins [possibly at time of collection]
----resulting in liver damage)

External Signs
Skin and fins
----Clamped fins (poor water quality, parasites)
----Sunken belly, thin (starved, bacteria, parasites, toxins, liver damage)
----White spots on skin/fins (lymphocystis, bacteria, fungus, dinoflagellates, parasites)
----Fish tissue extensions (various-shaped growths), abnormal growth from skin, fins, etc (hyperplasia,
tumor-benign or cancer)
----Dark spots in skin (embedded larvae of digenetic trematodes)
----Bloody spots or streaks or sores on skin/fins, lateral line pits (lymphocystis, bacteria, parasites,
----Bloated belly, scales raised from scale pockets (dropsy, kidney damage, tumor-benign or cancer)
----Body deformed, twisted, etc (genetic, injury in egg, vitamin/mineral deficiency, old age, etc)
----Fight (or collection) injuries on skin/fins; frayed fins/scale loss (injuries may lead to viral, bacterial, or
fungal infections)
----Increase in mucus production; whitish slime (bacteria, fungus, parasites, irritants, toxins)
----Easily visible living objects on skin (parasites, as: leeches, copepods, Argulus, isopods, Monogena)

----Cloudy (bacteria, toxins, poor water quality)
----Red, bloody (bacteria, toxins)
----Dark spots in eyes (embedded parasites, tumors)
----Eyes protruding or pop-eye (exophthalmos) (gas bubble disease, lymphocystis or parasites or tumor
behind eye in socket pushing eye out, bacteria)
----Eyes sunken (starved, bacteria, Mycobacterium)

----Flared (gill parasites, gill infection, toxin damage to gills, large isopod on gills)
----Gasping, irregular opercular beat, rapid opercular beat (bacteria, dinoflagellates, parasites, toxins)

----Pale gill color (viral or bacterial infection, parasites, toxins, internal bleeding)
----Spots on gills (lymphocystis, bacteria, parasites, dinoflagellates)
----Gill lamellae/filaments fused (bacteria, lymphocystis, hyperplasia)
----Gill lamellae/filaments eroded (bacteria, lymphocystis, parasites)

----Silver or gray or white sheen on skin (bacteria, Mycobacterium)
----Color changes, either getting lighter or darker (virus, bacteria, parasites, toxins)

----Filaments, nodules, growths, hyperplasia, etc on skin/fins (lymphocystis, tumors, bacteria, fungus,
parasites, irritants, toxins)

See these articles also:

Aquarium Ideas….. Collecting……........
Daily tank checks… Hyperplasia…......... Lymphocystis…...... Monogenea……..... Mycobacterium….. Necropsy……........ Parasites….............. Seeding……........... Stocking……..........
Substrate ingestion... Toxic algae…..........

Copyright 2006 by Dr. Adrian Lawler, Author Copyright 2006 by Aquarticles, Internet Sponsor

Author Information must remain with article.


Author: Dr. Adrian Lawler
Title: Some Signs of a Sick Fish
Summary: A comprehensive list of visual cues to identifying sick fish in aquariums.
Contact for editing purposes:
email: Adrian Lawler <>
Date first published: December 2006
Publication: Original to Aquarticles
Reprinted from Aquarticles: ARTICLE USE:
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Dr. Adrian Lawler,
P.O. Box 48,
Ocean Springs.
MS 39566
#205 - 5525 West Boulevard
Vancouver, British Columbia
V6M 3W6
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