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Common Freshwater Fish Diseases

One of the problems that many fish keepers face is the daunting task of identifying problems in the aquarium. Unfortunately, lack of experience, or subtle signs of stress can let a disease run rampant until it destroys the infected fish (sometimes the whole aquarium). I have included this page simply as a guide for diagnosing diseases in Tropical Fish. It is by no means complete, and I cannot guarantee that it is 100% accurate, but should help steer you in the right direction.

Always ask your local merchant if the medication you are buying is right for the disease you are trying to cure.




Fish may dart and gasp. Acidic Water Acid from fish wastes or decayed food.  Acidic water can iritate gills and skin. 
White water line. Plants die, fish have frayed fins. Alkaline water can destroy gill and fin tissues. Ammonia becomes more toxic. Alkaline Water Seashells or limestone in tank or hard water supply
Fish die. Water may be cloudy. Ammonia in Water Product of decaying wastes.
Threadlike worm hanging from fish. Base of anchor worm may be red. Anchor Worm A parasitic crustacean (Lernaea).
Quarter inch bubble like lumps slowly moving on skin and fins. Argulus A parasitic crustacean. Sometimes Ergasilus also.
Fish floats with fins closed. Betta Pinch Fin Gill parasites.
Small black specks on skin. Black Spot Small fish worms (Diplostomulum).
Red streaks on fins, especially goldfish. Parasites or Bacterial Infection Gyrodactylus parasites (monogenetic trematodes*), possibly bacterial septicemia
Fish dying within 24 hours. Gills become brown. Chloramine in Water Water company adding chloramine as a substitute for chlorine.
Fish dart about. gills pump furiously. Chlorine in water supply Chlorine added to water supply to kill bacteria.
Fins held close to body, especially in livebearing fish. Clamped Fins Gill or body parasites.
Water not crystal clear in established tank. Cloudy Water Wastes, small animal and plant life. Possibly over feeding.
Small particles suspended in water causing a cloudy look. Cloudy Water in New Tank Floating silts from gravel especially newly set up tanks.
White cotton sprouting from fins or body. Fungus, possibly Bacteria A bacteria, Flexobacter (Chondrococcus) or Fungus
Fish act very uncomfortable. Contaminated Tank water Chlorine, chloramine, copper or manufacturing oils in the water.
Grey film on all or part of body. External Parasite External protozoan parasite (costia necatrix).
Gills hang partly open at all times External Parasites (Dactylogyrus) Small gill parasites (monogenetic trematodes).
Fish darts about wildly and may smash into glass. Discus Head Worms Parasitic protozoa in sinus canals of head.
Fish bloated with scales standing out. Fish resembles a pine cone. Dropsy Kidney or Liver Damage (possibly both)
White film on eyes. Eye Cloud Eye injury. From handling or breeder fighting, possibly bacterial Infection
Fins ragged with jelly at edge of ragged part. Fin Fungus  This is not a disease exactly, but can be cured by improving water quality
Fins eaten away with redness at base of bad area. Fin Rot Sprolegnia fungus with Pseudomonas or Aeromonas bacteria.
Fish swim aimlessly. May have spasms. Bacterial Infection Internal Flavo Bacteria infection
Small lumps under skin or on fins. Flesh Worms Parasitic worms.
A light grey cottony growth or patch on any part of the fish. Fungus (grey) A parasitic fungus, Saprolegnia.
A white cottony growth or patch on any part of the fish. Fungus (white) A bacteria, Flexobacter (Chondrococcus) Columnaris.
Open sores on Gouramis or other anabantids. Bacterial Infection (Furunculosis) Aeromonas salmonicida bacteria.
Gills hang partly open and may pump fast. Gills may pump fast. Gills may appear red. Gill Flukes Small gill parasites (Gyrodactylus) monogenetic trematodes.
Fish rocks back and forth while staying in place. Gourami Disease Dactylogyrus parasites (monogenetic trematodes*).
Skin looks grey in patches. External Parasite Costia necatrix parasites.
Fish scrape on objects in tank. External Parasite Gyrodactylus parasites (monogenetic trematodes*).
Red steaks on fins usually near body with no sign of skin damage. Hemorrhagic Septicemia A bacteria, Aeromonas (liquefaciens) hydrophylia.
Tiny tree-like animals attatched to objects in tank. Hydra Carivorous microanimal
White sugar-like crystals on fins or body of fish, fish scrape against objects in tank External Parasite (ICK aka ICH, possibly others)) Ichthyophthirius multifiliis, a parasitic protozoan.
Fish are thin and act listless Internal Parasites or Internal Bacterial Infection Tiny internal parasites or Bacterial Infection (wasting diesease)
Fish may go to top or bottom or dash about when first introduced to the tank. Tank water is vastly different from original source water Change in water from which fish are first acclimated.
Stains on glass or ornaments. Iron In Water Supply Dissolved iron in water supply.
Insoluble white deposit or water line. Lime Deposits on Glass Caused by lime, a mineral found in water supplies or from salt buildup.
Fish stay in one place buy wiggle, wag, or shimmy. Livebearer Disease or Molly Disease Water lacks electrolytes needed for normal metabolism.
White cotton sprouting from mouth sides. Mouth Fungus A bacteria, Flexobacter (Chondrococcus).
Flesh eaten away around mouth. Mouth Rot Saprolegnia fungus with Pseudomonas or Aeromonas bacteria.
Color fades from red line. Line may turn yellow. Neon and Cardinal Disease A parasitic protozoan, Plistophora hyphessobryconis.
Fish dart about. Gills pump furiously after a water change. Dissolved gasses or metals in the water. Dissolved gasses or metals in the water.
Body skin of fish appears dusty in yellowish or grey patches. Oodinium (also called VELVET or RUST) A parasitic algae, Oodinium limneticum.
New Fish act stunned or gasp at top of water after releasing them from bag. Oxygen Deprivation, CO2 Poisoning, Ammonia Poisoning Ammonia and carbon dioxide suffocating fish in container.
White water lines. Plants die, fish have frayed fins. pH Bounces Back Up after Being Lowered Hard water supply. Seashells or limestone in tank.
Eyes stand out from sockets. Pop Eye Infection from fighting, possibly a bacterial infection causing fluid to accumulate behind the eyes
Flesh decays at the mouth or tail. Rot, Fungus Saprolegnia fungus with Pseudomonas or Aeromonas bacteria.
Frayed looking fins and moldy looking slime on body. Saprolegnia Fungus A true fungus called Saprolegnia invades the gills causing suffocation.
Small fish, especially neons, angels and guppies die without visible symptoms. Saprolegnia Fungus A true fungus called Saprolegnia invades the gills causing suffocation.
Fish swimming with head down (not level). Swim Bladder Disease Bladder infection.
Fish thin and weak. Parasites or Bacteria internal or external parasites. See GYRODACTYLUS. Possibly "wasting Disease" a bacterial infection
Water may be cloudy or have an odor. Too Many Dissolved organic Compounds Decaying food or plants.

Article written by Gareth (Aquafiend)

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