Parasites That Could Be Harming Your Tropical Fish

A Long Finned Surpe Tetra with an Investation of Aquatic Fungi

Have you ever heard of Ichthyophthirius multifiliis, otherwise known as ich? This protozoan parasite attacks nearly all species of fish in tropical environments and has been responsible for countless deaths among hobbyists who keep tropical fish.

In this article, we’ll cover seven other types of parasites that pose a threat to your fish, how they affect them, and what you can do to protect your pet from them!

Let’s take a look at these seven parasites harmful to your tropical fish.


Fin Rot

This bacterial infection is common in new and often young fish. The bacteria occur when poor water conditions like high ammonia or nitrite levels are present. This often occurs during a tank transition when fish are moved to a new aquarium with different water parameters than they are used to.

Fin rot can be treated and usually responds well to antibiotic treatment but can take several weeks to resolve fully. Reducing stress on your fish by ensuring they have high-quality food and clean, fresh water is also important to prevent fin rot from occurring again in the future.

A good way to reduce stress for your fish is to add plenty of hiding places for them so they feel safe. If you notice any problems with their fins (redness, discoloration) it’s best to treat them immediately before it spreads and causes further damage.

You should always quarantine new fish before introducing them into an established tank as fin rot can spread easily from one fish to another. Always wash your hands after handling any sick or dead fish!



Also known as dropsical disease or oedema disease, dropsy is a condition found in fish where fluid collects in their body cavities and other soft tissues. It can be caused by bacterial infections, genetic defects, and poor nutrition.

A common symptom of dropsy is lethargy; if your fish are sluggish and float near to the surface of your tank then they may have dropsy. Another sign is swelling around the eyes, gills, and fins which causes them to stick out from their bodies.

Other symptoms include loss of appetite, reduced activity levels, and scales that appear puffy or swollen. The main treatment for dropsy is antibiotics but there’s no guarantee it will work.

You should also try raising your water temperature up to 84 degrees Fahrenheit (29 degrees Celsius) which helps speed up metabolism and encourages your fish to eat more often.


Hole in the Head Disease

Hole in Head Disease (HHD) is an infection caused by several different species of worms. These live and breed in your fish’s mouth or gills and lead to large lesions appearing on their body.

Left untreated, Hole in Head disease can cause brain damage or even death. However, with proper treatment, it is easy to cure!

Here are some remedies for treating Hole in Head Disease.

This parasite lives inside your fish’s head, which is why it gets its name. The most common way for a fish to get HHD is from being stressed out due to bad water quality or overcrowding.

The hole in their head isn’t actually a hole but rather a large lesion that will eventually kill them if left untreated.



The most common of all fish diseases, hexamita is a parasite that comes from unsanitary living conditions. It’s not serious and your fish will probably recover on its own, but it’s important to observe how your fish are behaving and what their health looks like to determine if you need to treat for hexamita.

If you do decide to medicate, be sure to follow instructions carefully—too much medication can cause other problems. There are no over-the-counter medications for hexamita, so you should take your fish to an aquarium supply store or a veterinarian with experience treating tropical fish.

Antibiotics work best against hexamita. Keep in mind that treatment may involve two or three rounds of antibiotics over several weeks. You’ll also want to clean your tank thoroughly during treatment and possibly quarantine any new fish before adding them back into your tank.

While most cases of hexamita are mild, they can also lead to more serious complications such as liver damage or death if left untreated.



A flatworm that affects your fish’s digestive system. If infected with Chilodonella (pronounced kill-oh-den-eel-ah), your fish will exhibit symptoms such as loss of appetite and white stringy feces.

While generally harmless, the infection may lead to death if left untreated. Treatments include medication from a local tropical fish store. Keep in mind that medications should be administered with care; ask an employee for assistance if you have any questions about administering them correctly.

Also, never combine multiple treatments at once. This can result in fish becoming severely ill or even dying. Infected are often found swimming erratically near their tank’s surface. Other common signs of infection include clamped fins, lethargy, and rapid breathing.



The Costia parasite is a protozoan (one-celled animal) that feeds on mucus secretions and dead skin. They can live for up to four weeks in water and are completely removed with proper cleaning.

The Centers for Disease Control have found that Costia may be more common than previously thought among residents of both freshwater and marine environments, and are believed to spread through skin wounds or by ingestion.

In humans, they typically cause irritation but rarely cause serious illness. However, these parasites can be lethal to fish and should be treated immediately if discovered. Unfortunately, there is no known treatment for costia in fish; infected fish must either be isolated from other fish or euthanized.

Prevention involves practicing good hygiene—keeping hands clean before handling fish—and ensuring that equipment used to move fish between tanks has been properly sanitized after use.



This parasite can negatively affect your fish’s immune system and has been known to cause severe damage to gills, fins, and other body parts. It will eat away at scales on your fish’s body and is often identified by cloudiness in your water or a slimy film around your fish.

To treat it, remove these parasites with a freshwater dip or by adding salt directly into their tank. Keep an eye out for any new slime that appears on your fish; if you see any more signs of Trichodina, do another freshwater dip as soon as possible.

If that doesn’t work, then you may need to consult a veterinarian about additional treatments. You can also use anti-parasitic medications such as Praziquantel (which comes in both liquid and tablet form) to help rid your fish of these harmful creatures.

Always follow manufacturer instructions when using medications so you don’t accidentally overdose on your pet.

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