Healthy fish generally have strong immune systems and are capable of resisting most diseases, but when stress weakens the fish, the fish becomes more susceptible to illness. With ideal circumstances - proper water conditions, a diet including a variety of foods, uncrowded conditions, and an environment without stress - diseases rarely affect fish. Usually fish will only get sick when something in the environment is not right, or when new fish are added to the tank.
Fish have evolved to mask their illness if they are sick. They might look normal and healthy, even with severe disease issues. This evolutionary trait is a protective mechanism that limits the likelihood that a larger fish or predicator will spot them in a school of fish and attempt to catch them for a meal.
Spend time watching your fish, because you will see that they have specific biological functions or behaviors that change when they are distressed.
Changes You May See in Sick Fish
- Appetite - do the fish show an interest in food when it is presented?
- Alertness - do the fish seem alert and aware of their surroundings?
- Respiration - are the fish's breather rate fast (tachypnea) or slow (bradypnea)?
- Swimming activity - are the fish actively swimming?
- Color - are the fish their "normal" color?
Fish with diseases can show a variety of signs. If you notice your pet fish having any unusual disease signs, contact your veterinarian for further advice. Sometimes an illness can be treated with antibiotics put into a quarantine tank.
Common Signs of Disease
- Might appear disoriented, such as swimming upside down
- Leaving food uneaten, or not eating
- White spots on fins or body caused by the Ichyophirius parasite (ick)
- Discolored gills are usually caused by too much ammonia in the tank
- Trouble breathing such as gasping for breath at the surface of the water
- Bulging eyes are called Popeye disease (exophthalmia)
- Mucus on the body
- Rubbing on hard surfaces
- Isolation from the other fish in the tank
- Bloating. skin lesions, or sores anywhere on the body
- Crooked back or other changes in fish shape or size
- Parasites - white spot disease or ick, nematodes, anchor worms, fish lice, gill maggots, monogenean flukes
- Bacterial infections - columnaris disease and mycobacterial infections
- Fungal infections - Saprolegnia, Branchiomyces, and Fusarium
- Finrot or ulcer disease - caused by bacterial or fungal infections shows rotting of the fins
- Hole-in-the-head - holes in the front or side of a fish's head
- Fish pox - caused by a fish herpes virus resulting in gray, pink, or white wart-like growths (common in Koi fish)
- Velvet or coral fish disease - a parasitic disease that results in dusty and slimy scales (common in tropical fish)
- Ammonia or chlorine poisoning - resulting from poor water quality
Always quarantine all new fish before introducing them to the tank. If you put a new fish who is sick, into the aquarium, it's difficult to stop the disease from spreading.
The best defense is to use a cycled quarantine tank with all new arrivals and do not introduce them into the display tank until they have been quarantined for at least 4-6 weeks.
This time frame is important because fish can carry certain pathogens such as ich, but not show any symptoms for days to weeks. If they do need treatment for an illness, the time frame applies from when the treatment period ends. If you don't have a pre-cycled filter or tank, then quarantine usually isn't an option as you will end up cycling the tank with the fish in it, and the ammonia and nitrite created during the cycle will harm or kill most tropical fish.
Try not to purchase fish close to when the store receives shipments because the fish store is going to be dealing with the same possibility of sick fish with no symptoms. The stress of shipping increases the chances of infections.
Physical damage appears suddenly, so should be easy to tell apart from developmental abnormalities caused by poor genes or diet. Damaged fins occur when aquarium fish have been fighting, or with collisions against rocks, tank walls, or hood.
Common injuries for aquarium fish include nipped fins, missing scales, damaged eyes, abraded barbels, and dislocated jaws. The most common reasons for injuries in aquarium fish are handling, fighting, fin-nipping, and unsuccessful predation.
Chasing fish around an aquarium can cause them to rub against rocks or collide with the glass walls of the aquarium, making physical injuries likely. Even when netted it's easy for the aquarium fish's scales to be detached and fin membranes torn. Use the right net to corral aquarium fish - a fine net when chasing smaller fish, and a larger net for bigger fish. Place aquarium fish into a plastic container if possible and remove the fish using that container rather than a net.
The sudden appearance of wounds on the fish's head can occur with collisions against the hood or the walls of the aquarium. Keep fish in tanks that are the right size for them, and avoid exposing the fish to anything likely to make them jump - for example by turning the room lights on before turning the lights on in the aquarium hood. Although minor wounds will heal quickly, damage to the aquarium fish jaws is very serious, and such damage rarely ever gets better, and aquarium fish with damaged jaws usually need to be euthanized because they can no longer feed themselves.
Abrasions to aquarium fish are usually caused by the wrong substrate. Spiny eels, rays, and other burrowing aquarium fish will be damaged by sharp sand and coarse gravel. Secondary infections often follow on from abrasion damage, and these can be very difficult to treat successfully. Catfish and loaches kept in fish aquariums with sharp sand or coarse gravel often lose their barbels as well. Damaged barbels normally grow back quickly once the aquarium fish is moved to a fish aquarium with a safer substrate.
A fish's scales and fin membranes heal quickly, but if the fish’s damage is severe and there are obvious patches of blood or muscle visible, treat against bacteria and fungus.
Superficial damage to the cornea is visible as slight cloudiness. If aquarium water quality is good, eye damage can heal naturally. But under less than perfect conditions, eye damage often gets worse, leading to pop-eye and even the loss of the eye, but the loss of a fish's eye rarely causes any long-term health problems. Half-blinded fish may be disadvantaged at feeding time, particularly with predatory aquarium fish that hunt by sight.
Dislocated jaws are a common problem among aquarium fish that fight by pulling at each anothers jaws, especially when the 2 fish are mismatched. Unfortunately, there is no effective therapy, and if the damaged aquarium fish cannot feed itself it will eventually starve to death. Fish jaws are much too delicate to be "popped" back into position, though a vet may try to do so with very large fish. Observe the damaged fish, and if it cannot feed, it will need to be euthanized.