Fish Disease Index - Pictorial Guide

HLLE leonthepeon.jpg
Pictorial Guide to Marine Fish Diseases

Use this guide to find a picture that most closely matches your fish's symptoms. Click the link, if available for more detailed information on the disease and how to treat it.

Ich (Cryptocaryon Irritans)

One of the most well-known parasites in the hobby. It is identified by salt-like dots on the body and/or fins. This parasite can live for years in the gills of the fish and go unnoticed by the hobbyist for that time. Behavioral symptoms include breathing heavily, flashing, scratching on the sand or rocks and head twitching. Treatment can be done several ways including Copper, Tank Transfer Method (TTM), Chloroquine Phosphate and Hypo salinity. It’s important to note that hypo salinity will not treat for some strains of ick.

Photo: Rcfiftyone
Ich Rcfiftyone.jpg

ich unknown.jpg

Velvet (Amyloodinium ocellatum)

This is a very fast acting parasite. It’s usually identified by a large amount of very small, sugar-like dots, flashing, scratching against the substrate or rocks, rapid breathing and swimming into the flow of a powerhead. Bacterial infections often accompany velvet and can kill the fish even if swift treatment for the velvet is administered and successful. The only (currently proven) treatments for velvet are copper and chloroquine phosphate. Note that sometimes velvet will show no outward symptoms before the fish dies, so watching for the behavioral symptoms is key.

Photo: Flybill
velvet Flybill.jpg

Photo: Reefahholic
velvet Reefahholic.jpg


These are flatworms that are usually not seen until the infestation is severe. A freshwater dip will confirm their presence by turning the normally translucent worms opaque after 3-5 minutes in freshwater. Prazipro, general cure and formalin are effective treatments for flukes.–-general-guidelines.224423/
Photo: melypr1985
Flukes melypr1985 a.jpg

Photo: melypr1985
flukes melypr1985 b.jpg

Flukes unknown.jpg

Heavy Breathing
A symptom of parasites in the gills causing the fish dificulty breathing. It's a sign of flukes, ick, velvet and brook.

Black Ich (turbellarians)

Like black sprinkles over the body, this is another type of worm that attaches and feeds off of the fish. The same treatments for Flukes will work here as well. Prazipro, freshwater dips and formalin for the stubborn strains.

Photo: Fishyfingers
black ich Fishyfingers.jpg

Photo: FastFish
Black ich Fastfish.jpg

Piscicolidae Worm

Round worms that aren’t as common in our hobby, but can be treated with formalin dips. Once removed, watch for infection in the areas the worms were attached.
Photo: Alexis Terlep
Piscicolidae Worm Alexis Terlep.jpg

Dactylogyrus trematodes

These are a type of fluke or flatworm, but not one that we encounter every day. The usual treatments for flukes like Prazipro, General Cure, Freshwater dips and Formalin will all work for this.

Photo: Danny N
Dactylogyrus trematodes  Danny N.jpg


While this disease is often found in clownfish, it can affect any fish. It is often identified by an appearance of the skin sloughing off but can present in several different ways including ick-like spots or even almost look like velvet. This is a fast killer and needs to be treated quickly when symptoms are first spotted. Treatment includes Metroplex, Acriflavine, Freshwater dips and Formalin. Repeated treatments may be needed to clear the fish completely along with transferring the fish to a sterile tank in between dips.
Photo: 4theloveofish
Brook 4theloveofish.jpg

Photos: Unknown
Brook humblefish a.jpg
brook humblefish b.jpg

Bacterial Infection

These are very common and present in many ways. They can show as dark patches, white patches, torn or tattered fins, cloudy eyes, and red streaks or sores. There are two basic types of infections and each have their own challenges. Gram positive infections can be much slower acting, but can sometimes be difficult to spot before it’s late in the infection. Gram negative infections act quickly, sometimes killing the fish within 24 hours of onset. Treatment should be done with a broad-spectrum antibiotic or combining antibiotics for the widest spectrum possible. Combining Kanaplex, Furan 2 and Metroplex has the potential for providing the widest spectrum of antibiotics for a gram negative (or even a particularly stubborn gram positive infection) and can be combine safely together.
Photo: Orly20
bacterial infection Orly20.jpg

Photos: jocquill
bacterial infection jocquill b.jpg

Photo: cmcoker
bacterial infection cmcoker.jpg

Popeye – Infection/Injury

Identified by swelling of the eye or eyes. Typically, if just one eye is swollen, then it can be attributed to injury. When it’s both eyes, they will usually be cloudy and it’s either caused by an injury in one eye getting infected and it spreading to the other eye… OR a parasite like Flukes in the eyes which caused a secondary infection.

Photo: squalo_75
popeye squalo_75.jpg

Photo: terri_ann (injury + infection)
popeye terri_ann.jpg


A virus that presents as a small white spot, often confused for ich at first, that will stay in one place and grow in size. When larger it can look like cauliflower bits, usually on the fins, but can also be found on the body and face. It’s not typically dangerous unless it grows over the mouth or gills or internally on key organs. There is no treatment besides vitamins, low stress and pristine water conditions.

Photo: sbenus
lympho sbenus.jpg

Intestinal worms/Internal parasites

White, stringy poop indicates intestinal worms and can sometimes be accompanied by weight loss despite the fish eating well. This is a slow disease as long as the fish is eating. Soaking Metroplex in the food is the best course of action, but it can be dosed in the water as well as General Cure for a second choice of treatment.

intestinal worms humblefish.PNG


Common in clownfish, this appears as brown spots or ovals where a stinging coral or nem as come into contact with the fish’s skin. Typically nothing needs to be done to cure this as the clown becomes accustomed to it’s new host’s sting. This can occur with other fish, but again no action needs to be taken. It will heal up on it’s own.

HLLE – Head and Lateral Line Erosion

This usually starts out on the face, but not always. It will appear as the skin pealing from the face or eroding away. Holes (small or large) will appear on the face, head and along the lateral line of the fish. Good nutrition, clean water and low stress can help stop the erosion and sometimes heal it over some.

HLLE humblefish.jpg

Photo: LeonThePeon
HLLE leonthepeon.jpg

Uronema Marinum

Red soars that seem to kill quickly. It’s mostly (but not exclusive to) Chromis and clownfish. Treatment is difficult and can include Acriflavine, Metroplex, Copper or Chloroquine Phosphate.

Photo: Reefahholic
Uronema Reeferhholic.jpg

Rectal Prolapse

Identified by a protruding anus, a rectal prolapse can be caused by constipation, straining or even unknown parasitic infection. These things will usually correct themselves and adding vitamins to the food and dosing kanaplex can help with keep it from getting infected while it’s out. Premium water quality should be a priority during and after this time as well. On larger fish who don’t appear to be getting better, it’s possible to correct the issue yourself and use a purse-string suture to keep the anus inside the body until it’s healed.

Photo: mmarro99
prolapse mmarro99.jpg

Photos: M&M
prolapse M&M b.jpg
prolapse M&M.jpg

Distended Mouth -Locked open

This is a condition that can be caused by a physical trauma or if the fish bites (or comes into contact with) a stinging coral. Unfortunately, there is no treatment for this. It will either self-correct or it won’t.

Photo: cryotek74
distended mouth cryotek74.jpg


Fluid filled area of skin, that usually self-corrects. There’s no real treatment besides vitamins, healthy foods, clean water to help it along.
Photo: haleyf1024
abcess haleyf1024.jpg

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About author
Meredith Presley started keeping marine aquariums in 2007. She’s done everything wrong that can be done in the hobby (mostly but not all in that first year) and that has afforded her to learn a lot of hard lessons. Recently she’s been focused on marine disease diagnosis and treatment and hopes to focus on breeding soon as well. She also keeps a blog with basic info on saltwater keeping and her experiences with her own tank and livestock.

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