Common Freshwater Fish Diseases

Like any other pet, fish can get sick. And like any other pet, being aware of what is normal for your fish in terms of behaviour and appearance helps greatly to identify what could be ailing your tank inhabitants. The following are the most common fish diseases you may face as an aquarist, and what can you do about them.

Ammonia Poisoning/New Tank Syndrome

Ammonia Poisoning Symptoms: Red and irritated gills, fish gasping on the tank surface trying to breathe.

Ammonia poisoning or new tank syndrome happens when the bacterial colonies that act as biological filter and process ammonia on a tank are too weak or immature to deal with the tank’s biological load. We wrote a longer article about preventing and fixing new tank syndrome here. But in short, you will need to make daily water changes (or use an ammonia binding water conditioner such as Seachem’s Prime) until your tank has had a chance to cycle.

Freshwater White Spot Disease – Ick/Ich

(Ichthyophthirius multifiliis)

English: A velvet infected fish. Clearly visib...
English: A velvet infected fish. Clearly visible on head. (Photo credit: Wikipedia)

White Spot Symptoms: This is probably the most common fish affliction in freshwater tanks. Your fish may be covered in white spots, or look like they are covered in dust. They may be trying to scratch themselves against the walls of the aquarium or the decoration, as if they were itching.

Ick or white spot is easy to treat, and so common most aquarists will at some point encounter it. White spots are caused by a parasite that burrows itself into the fish’s skin and causes the spots and the irritation, but healthy fish usually can brush it off.

Velvet is a less common but similar parasitic fish disease that causes yellowish or greying spots, that often group together giving your fish a velvety appearance. The treatment is similar, just with velvet medication instead of Ick medication.

The parasites spend part of their lifecycle buried on your fish’s skin, and the other half floating around the tank trying to find a fish to attach themselves to. Treatment can only work on free-swimming parasites, as the ones on the fish are protected from the medication.

Treatment For White Spot Disease

In order to rid your tank of white spot disease or velvet you will need to eliminate all the parasites, which are only vulnerable during the part of their cycle when they are swimming. Raising the temperature of the tank (Slowly and gently) will speed up the parasite’s cycle a bit but you are looking at about a month of medication for Ick or Velvet.

Fortunately there are many Ick medications available, just make sure you use the correct dosage and remove your carbon filter as you don’t want it to remove the medication from the water right away. If you have scaleless fish such as loaches, you may need to halve the dose and treat for longer. Always follow the recommended treatment type even if your fish seem clear, as there may be parasites still in the water.

Parasites need a fish host to survive, so if you were to move all your fish to the hospital tank for treatment, the remaining Ick or Velvet parasites would be dead in about a month on a fish-less tank. Any tank that is not empty of fish will have parasites remaining for that time. White spot is a tank wide disease, so you will need always to treat the entire tank, even if all the affected fish have been removed.

Swim Bladder Problems

English: Ryukin goldfish with swim bladder dis...
English: Ryukin goldfish with swim bladder disease. (Photo credit: Wikipedia)

Swim Bladder Symptoms: Fish swims strangely, on the side or seemingly unable to remain at a particular depth in the tank. Fish may sink to the bottom of the tank, or float aimlessly to the top.

Fish’s swim bladder is the organ that allows them to float upright and choose the depth they want to swim on. It is actually a bladder full of air which the fish controls… until it gets damaged or affected by a bacterial infection or bloating.

Treatment for Swim Bladder Problems

So far, the fish keeping community hasn’t managed to reach a consensus on the best way to treat swim bladder affections, as the causes are varied and each fish is affected differently. However, the following are common causes and cures for swim bladder disease:

  • Fish digestive system is blocked: Try giving your fish shelled cooked peas (cheapest fish laxative ever) or reduce feedings for a few days.
  • Bacterial infection: If the peas fail, try using medicated food on a hospital tank

Sometimes the problem is caused by trauma during transport, in which case all you can do is keep the fish on a hospital tank until it has a chance to recover by himself.

Pop Eye

Pop Eye Symptoms: Your fish’s eye(s) bulge out of its head

This fish disease can be pretty scary. Sometimes it can be minor, sometimes it does look like your fish’s eyes are about to fall out of their orbit. This is often caused by bacteria or injury causing swelling of the eye. If it affects both eyes it is most likely bacterial infection, but if it only affects one eye it could be just injury (A fish’s equivalent to a black eye) often caused by transport.

Treatment for Pop Eye

If it looks like it could be a bacterial infection, start with checking the water conditions and treat with the right medication on a hospital tank. Aquarium salt may help the fish flush out liquids and reduce the swelling, but calm and quiet will go a long way towards helping him recover. Pay attention to your tank’s inhabitants and try to identify any bully that could be harming your other fish, as that could be causing stress and injuries.

Cloudy Eye

Cloudy Eye Symptoms: Your fish’s eye is not clear, and has a cloudy appearance.

The centre of your fish’s eye should always be clear, and the first signs of milkiness or cloudiness should raise alarm warnings: most likely a bacterial infection is affecting your fish’s vision.

If your fish has suffered an eye injury (see pop eye above) you may be dealing with a bacterial or parasitic opportunistic infection. However, if your fish just suddenly starts displaying a milky eye (or eyes) then the problem can be elsewhere.

Very old fish can, like humans, suffer from cataracts.

Cloudy Eye Treatment

As usual, it is a good idea to start with througly testing the water and specially the pH, as the wrong acidity or alkalinity is thought to cause cloudy eye on fish. A good diet, high in nutrients, can also help alleviate the symptoms and even clear cloudy eye completely.

If improving the water conditions doesn’t have a positive effect in a few weeks, or your fish is also suffering from pop eye, try antibiotic medication as a last resource.

Hole in the Head

Hole In the Head Symptoms: You can see holes in the head of your fish

Also known as lateral line disease, hole in the head disease is no joke. Sadly there is also no consensus on what is the exact cause, though the culprit seem to be specific pathogens coupled with nutritional deficiencies and overactive carbon filters that reduce the mineral content of the water.

Treatment for Hole In The Head

The only treatment for lateral line disease is improving the water conditions and changing the diet in order to give the fish a chance to recover. Try rich in vitamin flake foods, or even frozen food, to try and sort out any nutrient deficiency.


Photo depicting a Goldfish with Fish Dropsy (S...
Photo depicting a Goldfish with Fish Dropsy (Side View) (Photo credit: Wikipedia)

Dropsy Symptoms: Fish looks like a pine cone, bloated with the scales protruding outwards from the body.

Dropsy happens when fluid builds up in the fish’s body, as a result of an infection or parasitic disease. It’s a symptom and not a real disease, and can be caused by a variety of things, but you can still provide your fish some comfort while trying to figure out the underlying causes.

Treatment For Dropsy

Usually a fish affected by dropsy is in the later stages of a bacterial infection and has limited chances for survival. However, not all is lost. Move your fish to a quarantine tank, add a tablespoon of aquarium salt for every 5 gallons of water in the tank and try to medicate the feed using medicated food. The salt may help reduce the swelling, while the medicated food will help resolve the bacterial infection causing dropsy.

Fin Rot

Symptoms of Fin Rot: The fins and tail of your fish deteriorate and seem to rot slowly.

Early identification is key to survival so look for white areas appearing in the edges of the fins or tail, black or brown fin edges, fraying on the edges of the fins or tail. In severe cases, the entire fin could be rotten away leaving only reddened and inflamed tail rays.

Fin rot is usually a secondary symptom of a weak or stressed out fish. Bacteria invades the fins of your fish, usually from an injury, and just start eating away at them. This can happen on the find or on the tail, and will kill your fish unless rectified.

Treatment For Fin Rot

A fish affected by fin rot is a fish that is already battling stress, malnutrition or another disease. So your first course of action should be moving the fish to a hospital tank or correcting any less-than-ideal water conditions on your main tank.

Aggressive tank-mates are often a cause for fin rot, as they nibble at the tails and fins of more pacific fish, causing wounds that get infected and rot. Tail rot can proceed and make the body of your fish rot, which is a horrible way to die, so it is important to act as quick as possible.

Fish Fungus

Fungal Infection Symptoms: Fuzzy whitish or light grey patches on fins and/or body.

Most fish fungus won’t affect healthy fish, so this can be a symptom of a wider problem with the tank, such as inadequate tank maintenance or problems with the water quality. Fungus usually take hold when there is a pre-existing injury and spreads quickly. Every aquarium has fungal spores on it, they are everywhere, so you can’t really get rid of them, but the immune system of a healthy fish can fight them off easily.

Treatment for fungal infections in fish

You will need to use medication to treat fungal infections in fish, but only do so after ensuring the water quality in the tank is high and performing maintenance and a full checkout of the filters. You don’t want to spend a lot of money fixing the symptoms just for them to come back because the underlying causes aren’t fixed. This treatment is better performed on a squeaky clean hospital tank, as it can affect other fish in your display tank. Often, anti fungal fish medications also work for bacterial infections that may be the cause your fish defences are low.

Antibiotic solutions such as Myxazin, Melafix or Gentian Violet may help your fish recover quicker. Sometimes fin rot can be compounded by fungal infections, worsening the problem.

Many tropical fish diseases come down to water quality problems. Your first course of action if you suspect your fish is sick should always be checking the water quality and remedying any issues before trying more aggressive treatments such as antibiotics and other medications. If you end up using medication for fish, make sure to follow the instructions to the letter, as bad usage of medicines can make the problem worse and destroy the ecological balance of your tank.

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