How Do Aquarium Fish Get Parasites, Worms—What to Do

By Eddie Waithaka @aquariawise

How Do Aquarium Fish Get Parasites, Worms—What to Do

One of the most common challenges you will encounter while keeping ornamental fish in an aquarium is parasites and worms.

So, before you start keeping aquatic animals as a hobby, I recommend doing a little research on the disease-causing nuisances.

However, if you’ve already started keeping fish and are having to deal with parasites, this article will still be useful. It should help you understand pests better, plus give you a few insights on how to approach any occurrences.

To begin with, you need to understand precisely how parasites, including worms, get into your fish tank and affect your aquatic pets.

Well, unknown to many new fish keepers, parasites are present in your fish tank and on your fish at all times, albeit in small numbers. But because they are opportunistic, they often remain inactive, waiting for the ideal environment to multiply, such as when your fish is stressed.

Conditions that stress your fish in the aquarium are most time the catalyst that gets the process rolling, and when not remedied, culminates in your fish getting a systemic ailment.

The conditions include overstocking, low oxygen levels, improper water parameters, change in temperature or water ph, and poor diet.

That said, parasites (and parasitic worms) can also be introduced to your fish tank by new fish coming from an infected tank and feeding your livestock live-foods that carry parasites such as bloodworms, beef heart, brine shrimp, and many more.

See more insight below.

How Do You Know If (When) Your Fish Has Parasites

Parasites affecting tropical and ornamental fish are divided into two groups. Ectoparasites, which live on the outside of the host, including on gills, mouth, skin, and fin surfaces…

…and Endoparasites, which live in the tissues, blood, and organs, including the gastrointestinal tract.

Unless ectoparasites (external parasites) are too tiny to see with the naked eye or burrow in the inner layers of the host skin, most are easily identifiable as spots on a fish body.

The most common of which is ich, also called white spot disease.

For parasites that are not visible to the naked eye, the only other practical way to tell whether your fish is affected is to observe its (their) behavior.

Usually, an affected animal will rub on rocks, wood, and other decoration to relieve the irritation caused by the parasites. And in severe cases, you may even notice liaisons on your fishes’ skin.

A parasitized fish will also show general lethargy, including swimming slowly, hiding more than usual, breathing heavily and gasping, and have a poor appetite.

What To Do If Your Fish Has Parasites

Personallly, anytime time I have a fish affected by parasites, I follow these three steps: Quarantine, Natural remedies, and Medication.

See more details below.


Some parasites are transmitted directly from fish to fish, including some ectoparasitic protozoa and skin (gill) flukes.

As such, quarantining (hospitalizing) an affected fish could go a long way in making sure other aquatic animals in your tank are protected and remain healthy.

Setting up a hospital aquarium for the affected animal also gives you some wiggle room in case you need to treat the fish with commercially available medicines, which would otherwise harm members of your community, such as inverts.

Copper-based medicines, while not harmful to fish, are notoriously toxic to shrimp, crabs, and snails, thus only useable in small amounts in a community aquarium.

A quarantine (hospital) fish tank should not be too sophisticated. You’ll only need to make sure the water parameters are ideal for the sick fish by including the most crucial equipment, such as a filter and heater.

A 10-gallon with a sponge filter and a 50-watt heater should be enough for most tropical fish. Substrates, decorations, and plants are not necessary, but they won’t hurt if you have them.

Natural (Homemade) Remedies

Before you move to use harsh commercially available fish medicines, it’s best to try out natural treatments for aquarium fish diseases and parasites.

These methods, although not widely studied (scientifically), are less intrusive and have little to no residual effect on your fish or water chemistry.

Plus, most homemade fish treatments will work on many different species of tropical fish kept by hobbyists, such as goldfish, cichlids, betta, gourami, mollies, guppies, tetras, swordtails, cories, platys, and plecos.

The idea behind their (natural organic fish treatments) is to nature a strong-immune-system and provide a healthy environment for your fish, but unideal for opportunistic parasites that target unhealthy or stressed aquatic pets.

For instance, salt dips and baths will remove external parasites (such as Ick), bacteria and other disease-causing organisms on affected animals, and new fish, before (re) introducing them into a community tank with healthy animals.

Fresh water bath for saltwater fish and saline water dips for freshwater fish are part of the remedies you should consider initially, though there are a host of other products you can use.

When treating external parasites, a dip is the method of choice. Place four teaspoons of salt in a clean bucket, then slowly add one gallon of water from the aquarium while swirling to dissolve the salt. Once completely dissolved, place your fish in the bucket for five to 30 minutes depending on the species affinity to water salinity: The Spruce Pets.

Permanganate will eliminate anchor worms, fish lice, fluke, ick, cottonmouth, fungus, and many bacteria. Only make sure to mix it with distilled water (in recommended concentrations) for optimal potency and efficacy.

Natural antiparasite vegetables, fruits, herbs, and other safe ingestible or food ingredients are efficient when treating your fish of internal parasites.

More than often, I’ve heard and seen some hobbyists even feed their fish fruits.

Though I recommend using them as a component in your homemade fish recipe to both make sure your aquatic animals only ingest allowable amounts and to keep them from decomposing in your tank and causing a filthy mess.

Having said that, please note that different species of fish react differently to natural treatment, so make sure you thoroughly research🧐 any remedy you intend to use on your fish.

Medicate Your Fish

Sometimes, the only option you have of saving a fish affected by parasites is to medicate, especially when the infection is too far gone.

Fortunately, if you already isolated the affected fish, you might not need to add chemicals in your main tank, meaning the rest of your animals won’t be exposed.

Make sure you hospitalize all affected animals, including the ones you suspect, might have caught the parasite from the original host.

If you are a little lost on what medicines to use, worry not because most commercially available treatments now are more broad-spectrum, which means that they can treat many different types of organisms that cause diseases.

As an instance, methylene blue is effective aganist skin and gill fluke, Ick, velvet, fungus, and many external parasites and bacterial infections.

Plus, it’s safe to use for many aquatic invertebrates.

Formalin and malachite green, often used in combination, rid your fish of all fungus infections, which albeit not disease-causing organisms, can be a pain in the neck.

How to Treat Internal Parasites in Aquarium Fish

Internal parasites are simply bugs that are inside of your fish, that basically start to eat your fish from the inside-out. They are a common killer of finnies, with quite a few symptoms that give you an idea your pet is affected.

Hiding, poor appetite, and lethargy are some of the signs to look out for, though white poop is unique but usual with internal parasites.

Most times, it will be a stringy-white poop that will be hanging out of your fish.

One other unique sign with internal fish parasites is a sunken-belly.

While an affected fish will eat aggressively, it will get skinny, and the stomach gradually sink-in because all the food is consumed by the parasites.

While there are many schools of thought, I prefer usingParaguard by Seachem more as a preventative treatment.

So, if you are just starting to see a few signs and need to treat your entire tank, I recommend using it.

If you decide to use other alternatives such as powdered medication, I advise you to start by isolating the affected fish. Otherwise, you will end up using too much of it, even when it’s not necessary.

Moreover, instead of medicating the entire tank, it’s better you add the treatment to your fish’s food. This way, the treatment goes directly to where there is the need and immediately impacts the parasites.

You may also want to add Epsom salt to your tank, which is a natural laxative that cleans out your fish’s system. Use a tablespoon of salt per ten gallons of water.

As we saw, Paraguard is perfectly fine for finnies that just started showing signs of parasitic attack. However, if it’s severe and you’ve got fish weathering away, you should consider something much stronger.

The two medication I use, which are straight to the point and will sufficiently sort your fish out are API General Cure and Metroplex.

Both treatments are readily available in pet stores, easy to use, and quite efficient with almost all tropical fish species.

Mix the remedies into your fish food, especially their favorite treat, because the med makes the food somewhat nasty. If you use options that your fish don’ enjoy, you may not achieve the intended goal, at least not as fast.

How To Get Rid of (Keep Out) Parasites in Your Fish Tank

Once you’ve sorted out your parasite mess by hospitalizing and treating your fish, you may want to make sure there is none left in your tank, plus keep them out for good.

So, how do you achieve this?

Well, for starters, note that parasites will always lurk in your fish tank, which is perfectly fine. Only make sure the numbers are contained, and the pests are not active to the extent of harming your fish.

Below is what you need to do!

Good Water Quality, Chemistry

As I mentioned, parasites are opportunistic, meaning they thrive when the conditions are right.

Overall, anything that affects your fishes’ well-being and increases their stress level is a catalyst for parasitic infections. This includes poor water quality and the wrong water chemistry for the fish you are maintaining.

In regard to water quality, you want to make sure that the ammonia and nitrite levels in your fish tank remain at zero ppm and nitrate readings don’t rise above 20ppm; 10ppm would be much better.

Cleaning your fish tank, performing water changes, investing in a good filter, and conditioning your tank are all efficient ways of making sure your aquarium remains pristine and free of harmful parasites.

Moving on to water chemistry, be sure to keep an eye on trace minerals and metals, especially in tap water, to ensure nothing that would harm your fish gets into the tank.

Chlorine, phosphates, iron, and copper from municipal water sources are of particular concern and should be kept at very minimal levels.

The GH (general hardness) and KH (Carbonate Hardness) of the water should also be ideal for the fish your keeping. Water with too many dissolved minerals tends to have an alkaline ph and is only ideal for fishes like cichlids that prefer things on the basic side.

You do not want to have fishes that prefer soft water with an acidic ph, such as those native to the Amazon ecosystem, in hard, alkaline water. Because, though they won’t necessarily die, they won’t be of optimal health, meaning they are more susceptible to parasitic infections.

Dietary Requirement

Most tropical fishes are not hard to feed. They largely accept a wide variety of food available in fish stores, depending on what each species prefers.

However, some fleshy foods, including live ones such as bloodworms and meats, such as beef heart, are known to carry parasites that are transferable to fish.

Most harmful bacteria and other pathogens affecting feeder fish or insects you are using as live foods will also be passed directly to you are aquatic pet if you are not careful.

Remember that most live foods do not last as long as the dry alternatives, such as flake or pellets as well. Meaning if you feed them to your fish after a long while, they will very likely have gone bad, putting your animals at risk.

So, research on your fishes dietary requirement, including those with sensitive digestive systems such as cichlids, and feed them only well cultured live foods, and any other type that risk passing worm or parasites to your livestock.

Quarantine New Fish

New fish are notoriously infamous for introducing parasites into a new tank when coming from infected water, which is quite common in pet stores community tanks.

Regardless of whether you a doing a five-dollar tetra or an expensive fish like discus or some cichlids, one fish can bring in a disease (parasite) and wipe out your entire population😢. So, some form of quarantine is crucial.

As such, it is always a good practice to quarantine your new members for between 2 and 4 weeks before introducing them to your main tank.

During that period, observe them for any ailments and treat them while including preventative remedies, such as salt baths, temperature regulation, and conditioner.

Rememeber to feed them their preferred diet as well, plus perform regular water changes and clean the tank where need be.

The great thing about a quarantine tank is that it’s not too demanding.

Plus, apart from treating your fish off any parasite or pathogen it might have, you also get a chance to gradually bring your fish up to the settings in the aquarium you plan to add them in.

Just remember to add a filter in the quarantine tank and a heater while keeping tropical fish that prefer things on the warmer side.

Treat Ailments to Prevent Secondary Infections

We’ve already seen that parasites prey on stressed fish more than healthy ones. As such, a sick fish with a weakly immune system is prone to secondary attack if a primary ailment is not correctly treated.

This includes fungus, bacteria, and even parasites like ich.

From experience, I learned it’s best to keep treating your fish for an extra week or two even when you are confident the ailment is gone. This will keep your fish safe and keep any opportunistic illnesses at bay.

If you fail to take such precaution and your fish remains stressed with low immunity, you might keep battling parasites without much success.

Happy fish 🐠🐟🐡 keeping, see you on the next one.

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