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How to Identify and Treat Common Diseases in Freshwater Tropical Fish

Fin Rot on Betta

Tropical fish, like any other living organism, can become diseased and may need some form of treatment.

Are there common diseases amongst freshwater tropical fish? Let’s identify the most common and also how to deal with them.

An aquarium is a totally enclosed world. In some ways, you can compare a school of fishes in an aquarium with a school of children. If one child goes to school with a cold then, very quickly, many of the children in the school will catch that cold.

In your aquarium, because it is an enclosed environment, if one of the fishes catch some form of illness and if the illness is contagious then it may be only a matter of time before many of the fish catch that same illness.

As an aquarist, you need to be ever-vigilant for signs of ill-health amongst your fishes and act swiftly to mitigate the risk of contagion.

The good news is that your aquarium includes an efficient filtration system which can eliminate many water-borne illnesses before they become an issue. The less good news is that, at some time, you are likely to witness illness amongst one or more of your fishes.

In this article, we’ll have a look at some of the more obvious diseases and illnesses that you are likely to witness in your aquarium at some point and determine the most appropriate course of treatment.


One of the most common causes of malady in an aquarium is overfeeding of the fishes. When you feed your fish, such food should be completely eaten within a few minutes. Excess food will fall to the bottom of the tank and start to rot and this process will provide a breeding ground for bacteria to form.

In the wild as well as in your aquarium, many species of fish will feed off plants and algae in the tank.

If you find that there is any build-up of excess food on the substrate of your aquarium then use a tank vacuum (see Additional resources) to remove the debris and feed your fish less food in the future.

On Discovering a Dead Fish

Since fish are living animals then it is inevitable that fish will die in your aquarium. This is something about which you must be both vigillant and diligent.

If you find a dead fish then remove it from the aquarium and examine it for the possible cause of death. In particular, look for any signs of any of the contagious diseases listed below.

At the same time keep a very close eye of your remaining fishes, looking for signs of contagion within the tank.

If in doubt, apply a general cure (see Additional resources, below) to the tank the better to protect your aquarium

Ichthyophthirius multifiliis – “Ick” – White Spot

White Spot

“Ick” or white spot is a parasitic protozoan, which manifests itself as tiny, white spots on the body of the fish. In my experience, you are most likely to encounter “Ick” if you introduce new fish into your aquarium, especially if you don’t take the time slowly to equalize the temperature from the water in which the new fish is contained to the temperature of the water in your aquarium.

You may observe a fish scratching itself against the substrate or a hard object in the aquarium and this may be a good sign that the fish has “Ick” before you ever see any white spots on the body of the animal.

When the parasite reaches maturity it will be around 1mm in size. It will fall off the fish and divide into several hundred to several thousand new parasites which will then seek out new hosts within the tank. The photograph above shows a fish in an aquarium where “Ick” is well established.

There are many commercial treatments now available to treat an aquarium in which “Ick” has been discovered. Essentially, such treatments will treat the tank as well as the fishes within it. Treatment serves to prevent the organism from reaching a new host whilst, at the same time, will treat and protect the diseased fishes. Follow the directions provided by the treatment manufacturer.

Fin Rot

Fin rot is generally either a bacterial infection of fishes or can be caused by fins being nipped by other fish in the aquarium (see featured image at the top of this article).

First, it is best to establish whether one fish or several fishes are affected by fin rot. In my experience, I have found that Gouramis have a tendency to nip the tail fins of other fish, particularly those with trailing tail fins such as Fantail Guppies or Betta Splendens. Be observant for signs of tail-nipping fishes.

If the cause is bacterial then it may be that you are overfeeding your fishes so check for signs of overfeeding on the substrate of your aquarium and, if necessary, remove any build-up of decaying food material.

You can add a fin-rot cure to your tank and increasing the vitamin C in the diet of your fishes can help.



Dropsy is a disease which appears not to be contagious. The affected fish appears to bloat up and its scales stand out.

There is no specific treatment for dropsy but you may wish to try a general cure. If this does not work then it is kinder to destroy the fish rather than have it suffer until it dies.


Flukes are a parasitic flatworm that can affect the skin and gills of fishes. Symptoms can include the fish dashing wildly around the tank before coming to a sudden stop, apparently exhausted. This suggests that the fish is in considerable distress and needs your help.

Flukes are best treated using something like the general cure linked to in the additional resources below.


Hole-in-the-head is caused by the Hexamita parasite and particularly affects Discus and Cichlids.


The parasite is identified by pale and ulcerated areas around the head and the condition may be treated using the general cure linked to in the additional resources below.

Swim Bladder Disease

In my experience, I have found swim bladder disease to be the most prevalent amongst catfish. The swim bladder is an internal organ of the fish which enables the fish to maintain its orientation in the water together with its buoyancy, thus enabling it to remain static without making an effort so to do.

It is generally thought that the malady may be caused by an internal parasite but it is also believed that either overeating or constipation may cause the condition.

Swim bladder disorder is easy to spot because an affected fish may swim upside down or “roll around” rather than maintaining its usual equilibrium.

It would be sensible to isolate the fish by placing it in a breeding box in the aquarium and not feeding it for a few days to see if the condition goes away. Using a general cure may also help, as there may be some other underlying problem

Isolation chamber

In terms of any malady observed in your aquarium, it may be wise to have an isolation box prepared. Generally speaking and depending on the size of fish that appears to be ill or infected, a breeding box will suffice to isolate the affected fish thus affording better protection to the remaining fishes in the aquarium.

It is recommended that the aquarium is treated with the appropriate remedy for the identified malady and then the specifically infected fish can be placed in the isolation chamber which is filled with water (containing the remedy) from the aquarium but with the top of the isolation chamber above the level of the water in the aquarium.

In the event that the malady is infectious then, if you catch it early, you may lessen the chances of infection in the general population within the aquarium, as the affected fish is isolated.

Tropical fish diseases – Video

Featured image showing fin rot courtesy of: Dizzy Respect

Mike Wheeler

I started keeping freshwater tropical fish in 1972 and it has been something of a passion ever since. In this website, my aim is to build up an everyman's guide to help the everyday aquarist get the best from this inspiring and entertaining hobby.

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