Common Fish Diseases
This article was taken in part from http://www.versaquatics.com

Ammonia Poisoning
Symptoms: Red or bleeding gills. Fish may appear darker in color and may gasp for air at the surface.

Bacterial Gill Disease
Symptoms: This disease is difficult to accurately diagnose visually. Fish may breath heavily and show bright red gills. Fish may sit on the bottom of the aquarium or near filter returns at the water surface.

Gill Worms
Symptoms: Rapid breathing accompanied by gasping at the surface of the water or sitting on the bottom of the aquarium. Worms may be seen hanging from the gills. Gills undergo severe damage leading to suffocation and death.

Mouth Fungus/Saddle Back Disease
Symptoms: Although commonly called mouth fungus this infection is really a bacterial infection. The bacterial colonies form raised grayish patches on the fins and mouth area of fish. Live bearers such as Guppies and Mollies develop grayish patches on their back giving rise to the name "saddle back disease". The infected areas may develop into red ulcers and also infect the gills causing rapid loss of fish.

Hole-ln-The-Head Disease
Symptoms: The most obvious symptom is the pitting and erosion of skin and muscle tissue around the face of the fish. The erosion of the tissue appears to be a symptom of the parasite in the intestine - not on the surface of the fish itself. Many fish simply show poor appetite, weight loss, nervousness, and ultimately death.

Fish Lice
Symptoms: Its easily identified by it's round flat shell. The parasites often can be seen moving about on the fish. The piercing mouth part damages the skin leading to secondary bacterial and/or fungal infections.

Anchor Worms & Skin Flukes
Symptoms: Can be identified as a grayish worm attached to the fish with two egg sacs at the opposite end. Tissue is often red at the point of attachment. Secondary bacterial/fungal infections may occur. Skin flukes are not visible without the aid of a microscope. Fish will scratch against objects in the aquarium and appear listless. A grayish film may form on the body. Secondary bacterial/fungal infections are common.

Neon Tetra Disease
Symptoms: Parasites infect the muscles of the fish resulting in loss of color and a deformed appearance of the fish. Fish loss can be high. Treatment is not always successful.

Fin and Tail Rot
Symptoms: Fins appear ragged and split. Disease can progress completely eroding fins and tail to the body of the fish. Secondary fungal infections commonly occur.

Dropsy and Malawi Bloat
Symptoms: Fish develop a bloated appearance due to build-up of fluid in the body cavity. Scales may appear to stick out from the sides of the fish. In advanced cases goldfish lose ability to swim and float upside down.

Eye Cloud, Pop Eye, and Body Slime
Symptoms: Eyes develop a whitish haze and/or protrude from the head. Fish's body develops hazy or slimy patches on the body. If parasitic, fish may scratch on objects in the aquarium and exhibit rapid breathing.

Furunculous
Symptom: Fish exhibit open red sores on the body.

Fungus Infection
Symptoms: True fungal infections appear as whitish cottony tufts or patches on the mouth, skin, and fins of tropical fish, goldfish, and Koi.

Velvet
Symptoms: Appearance: Parasites on the skin cause a golden velvety appearance on the sides of the fish. Gills are also attacked causing labored breathing and scratching on objects in the aquarium.

Bacterial Hemorrhagic Septicemia
Symptoms: Fish show blood streaks in the fins and body.

Subclinical Parasitic Infestation
Symptoms: Fish may simply scratch on objects in the aquarium or develop sliminess of the skin or labored breathing.