Intro to Fish Diseases

Dealing with a sick fish is never fun, but it is an unfortunate reality of keeping aquariums.  Not knowing how they got sick, whether they will survive or whether they will infect the rest of the tank can be stressful in an ordinarily stress free hobby.

Many diseases can be prevented through proper tank maintenance, care and adequate water changes.  But there are just as many illnesses that can’t be prevented by simply taking proper care of your aquarium.

Symptoms of Fish Disease

Below is a list of common symptoms that may be observed in a fish that isn’t doing well.  Many of these can be obvious while can be very subtle.


  • Eyes are bulging or missing
  • Eyes are cloudy
  • Eyes and/or areas around eyes are discolored


  • Fins look ragged or are missing
  • Fins are splitting
  • Fins appear stuck to the body


  • Fish color is fading or not as vibrant as it once was


  • Obvious weight loss
  • Swelling on the body
  • White bumps on the fish’s body


  • Lack of appetite
  • Inability to swim straight
  • Fish is inactive or laying on decorations or the aquarium floor
  • Fish won’t leave the surface
  • Fish gills are very obvious

Treatment of Fish Disease

Water Parameters and Malfunctioning Equipment

So you think your fish is sick and you now must decide on a course of action.  The first things to check your water parameters, do a small water change, and then check that all of your equipment is functioning properly.

Test all of your water parameters.  Are the ammonia, nitrites, nitrates, and pH all normal?  If not, do a water change any pay very close attention to your fish and the parameters over the next couple days.

Check the water temperature.  Is it within an acceptable range?  Is your heater working? Is it too hot?  Too cold?

Is your filter still running?  Is it clogged and need to be cleaned? Keep in mind it should be running 24 hours a day, 7 days a week.

Have you added any new decorations that aren’t made specifically for an aquarium?  Did you put in new decorations without cleaning them?  If you’ve put something in your tank that isn’t specifically made for aquariums, it may be giving off something that is toxic to the fish.  Remove the item and do a large water change.

If any of the above are not normal or not functioning properly, start there.  Replace your heater if needed, clean your filter if water isn’t flowing correctly, etc.

If everything is functioning properly or if you have fixed any broken equipment and it has been several days, you need to move onto the next step.

Common Ailments, Their Symptoms and Treatment

Fungal Diseases


Saprolegnia will appear as small fur-like objects on the body.  They will often be white in color and some may beat floating in the water.  Do a large water change and then treat the tank using a potassium permanganate medication.

Egg Fungus

Egg fungus will appear as a growth on laid eggs. When it is recognized, the eggs must be removed from the tank as soon as possible.  The fungus will quickly infect other tankmates.  Once the eggs are removed, do a large water change and keep a close watch on your fish over the next few weeks.

Bacterial Diseases

Fin Rot

Fin rot is characterized by decaying of fins, most often the tail fin and the dorsal fin.  This is often accompanied by a fish that has been nipping at these fins.  The nipping may begin before symptoms appear and will likely continue.

To treat fin rot, do a large water change, remove any fish that bullying the diseased fish, and treat the water with an antifungal medication.

Debivort [CC-BY-SA-3.0]
Debivort [CC-BY-SA-3.0]

Swim Bladder Disease

The symptoms of swim bladder disease are most often a change in behavior.  Fish with swim bladder disease will often be seen struggling to swim upright.  They may be floating at an awkward angle.

If water parameters are all in check, lower the water level about halfway.  Keep a close eye on the fish over the next few days for symptoms of any other diseases and treat as required.

Mouth Fungus

The symptoms of mouth fungus are a white furry growth around the mouth, a lack of appetite, and quick jerking movements.

To treat mouth fungus, do a large water change and treat using an aquarium safe antibiotic.

Piscine Tuberculosis

The symptoms of piscine tuberculosis include bulging eyes, loss of color and appetite and significant weight loss.  If you suspect one of your fish has piscine tuberculosis, remove it from the tank immediately.

There is no treatment and it is extremely contagious.  If it spreads after you’ve removed the diseased fish, all fish must be removed (and are likely to die).  Do not put them in any tank with other fish that haven’t been exposed to the bacteria. The ENTIRE aquarium must be completely cleaned and sanitized before new fish can be added.  It may be easier to replace the filter media and substrate rather than sanitize it.


Pseudomoniasis will appear as ulcers on the sides of the body and around the mouth.

Pseudomoniasis should be treated with antibiotics.  However, many strains have evolved to be resistant to antibiotics.


Vibriosis has several symptoms. Look for swollen stomachs, skin discoloration, and a lack of energy from ordinarily active fish.

Vibriosis is treated using an antibiotic.

Viral Diseases

Malawi Bloat – Malawi bloat typically only shows up in African Cichlids from Lake Malawi.  Malawi Bloat symptoms include bulging eyes and enlarged stomachs.

It should be treated by providing a high fiber diet.


Lymphocystis symptoms include white growths on the body and face.  The sick fish should be removed from the tank and given several days to recover.

There is no effective treatment other than to give your fish the best water parameters possible.  Keep a close watch on the rest of your fish as it can be contagious.


The symptoms of iridovirus include a lack of appetite, lethargy, and a change in color.

There is no treatment and it is very deadly.  Remove the fish from the tank and hope for the best.


Ichthyophthirius multifiliis (Ich)

Ich appears as small white spots all over the body.  They can be very dense or very sparse.  This is one of the most common diseases seen in aquariums.

To treat ich, raise the water temperature approximately 8-10 degrees to speed up the lifecycle of the parasite, add approximately 1 tbsp per gallon of salt to the aquarium, and add a commercially available ich treatment.

Djpalme [Public domain]
Djpalme [Public domain]

Anchor Worms

Anchor worms will appear as small white fibrous worms up to an inch long hanging off the side of the body.

They can be treated by physically removing the parasites from the fish.  Take a wet towel and hold the fish in your hands and pull off the anchor worms with tweezers.  Treat the water with an insecticide.


Fish lice will be easily seen with the naked eye.  They will appear as small white bugs that can sometimes be seen crawling around the fish’s body.

Treatment of fish lice is very much like anchor worm treatment.  Hold the fish in your hands with a wet towel and physically remove the lice with tweezers.  Search the aquarium and remove any other lice that can be seen. The aquarium should then be treated with an insecticide.

Hexamita (Hole-in-the-Head)

Hole in the head symptoms include the development of sores on the head.  It’s typically seen in discus fish.

Hexamita should be treated with miconazole medication.


Flukes are most often identified by a fish’s difficulty breathing.  It may be floating near the surface at an awkward angle.  It’s gills may be moving erratically.  The fish may rub its body against the decorations, substrate, or glass.

Flukes should be treated with a commercially available medication specifically for flukes.

How to Apply Treatment

Follow the instructions carefully for any commercial medication you are going to use. Some fish can not tolerate certain medications so double check that the medication is compatible with ALL of the fish in your aquarium. Remove any carbon from your filtration system as it will neutralize most medications.