8 Common Fish Diseases and How to Treat Them

Reviewed By Tom •  Updated: 07/13/23 •  6 min read
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Does your fish have an unknown disease that is affecting its health? These 8 common fish and marine life diseases all have easy to obtain solutions.

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Keeping an aquarium or fish tank is a wonderful way to keep pets. They both provide entertainment in their chosen room and can add color to a space. As well as being entertaining, keeping a good-sized tank gets those pets out of pet shops and into homes. Aquarium keepers become collectors, ever chasing the perfect color, shape, or size of fish or marine animal. Unfortunately, every time you add a new fish to your tank, you risk brining in the types of diseases that can harm your other fish. This article covers 8 common diseases and how you can treat them before they infect your other fish.

8 Common Fish Health Concerns and Treatment

Whether you have a freshwater tank or a saltwater tank, these fish and marine life diseases could eradicate your aquarium if you leave them untreated. Know the signs before they happen to your fish.

1 – The Ich

One of the worst diseases affecting both freshwater fish (Ichthyophthirius) and saltwater (Cryptocaryon irritans) fish is Ich. You might know this parasitical illness better as “White Spot,” named after the symptoms it produces. The fish become covered in white spots, which usually begin on the fins. You treat white spots with a dedicated medication found at most pet stores. Look for an anti-parasite water treatment. Pay extra care to your tank’s cleanliness for a whole month afterwards to keep any offspring from hatching.

2 – Clown Fish Disease

This disease is so common among clown fish that it got its nickname from them. The real name of the disease is Brooklynella. The Ich brings your fish out in small white spots which often appear first on the tail. Clownfish disease also turns your fish white, but it is more like a blotchy film than white spots. Your fish will look blotchy, as if it were swimming through cobwebs, rather than spotted. 5-minute freshwater dips can help, as can bathing your fish in chloroquine phosphate or similar medicinal baths. You may avoid this disease entirely if you keep the correct fish out of your aquarium. AquAnswers have excellent blogs to help you choose fish for your aquarium.

3 – Parasites

The Ich is just one example of a parasite which attaches itself to your fish. Parasites require a living host and will slowly kill your marine life. Parasites always come into your aquarium when you introduce new fish. You can prevent the presence of parasites by properly quarantining new fish before they enter the tank. Otherwise, each parasite will have a different medicinal treatment.

Fish parasites include:

4 – Lymphocystis

Lymphocystis is a disease which appears as wart-like lumps on your fish. First described in 1874, it has since appeared all over the world. The cause of the disease was not apparent until the 1960’s, when scientists identified it as a bacteria. Found in 125 known species, this disease is one of the gentler lymphocystiviruses because it doesn’t necessarily kill the fish. It does make them less pretty, and, in severe cases, the fish may pass away.

5 – The Black Ich

This disease is what it sounds like. The infection spreads by a type of flat worm which attaches itself to marine life. It will look like black spots instead of white spots, and they will form as small dots. The spots rise in bumps, which is your clue that this is the black ich and not just Flukes. These are similar parasites which make fish lethargic. The danger with both parasites is that they cause bacterial infections after the fact. It is these infections which cause further complications and can kill the fish.

6 – Polycystic Kidney Disease

This is a common killer among pet goldfish. Whether in your aquarium or in the garden pond, cysts can form on your fish’s kidneys. Unfortunately, this one is a terminal disease. There are no kidney transplants for goldfish. Instead, you must keep the fish as comfortable as it can until it passes away. You might never even know that the fish is sick until it is in the end stages and refuses food. In most cases, your fish’s stomach becomes distended as the kidneys enlarge, swelling to around 6.3% of their normal size.

7 – Cloudy Eyes

Cloudy eyes have many causes. Just as the name suggests, the symptoms see your fish becoming cloudy eyed. This will eventually impact their vision until your fish goes blind. The problem may be due to parasites or bacteria which have gotten into your aquarium. This disease might also result from old age, a poor diet, or excessive stress. If your fish is still young, quarantine your tank for a week or two and give it a deep clean. You might also investigate where you store your fish during a water change, as it may be causing stress.

8 – Fin Rot

The most likely thing your fish will catch, fin rot often arrives in your tank when infected fish are added. Common in pet stores where large numbers of fish stay together, this disease easily passes between species. Fin rot degenerates the fins, leaving bloody trails in them which may or may not heal after treatment. If the red, bloody infection reaches the base of the fin then the fish is likely to die. You can pick up a specific water treatment which kills off this infection at most pet shops.

Is Your Fish Sick with Another Disease?

If none of the above diseases, infections, or health issues are bothering your fish, but your fish is still sick, you can take them to the vet. Obviously you would struggle to keep a heated aquarium in your car to drive to your nearest animal hospital, but you can consult a vet online. An animal health professional will give you further advice on how to keep your expensive fish alive.

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Tom has always loved to write since he was little - he wanted to be either a writer or a veterinary doctor, but he ended up being a professional writer while most of his works are based on animals. He was born in San Francisco but later moved to Texas to continue his job as a writer. He graduated from the University of San Francisco where he studied biotechnology. He is happily married and a soon to be father!