Black Worms in the Fish Tank – Identification and Solutions

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Some hobbyists feed live worms to their fish from time to time as a tasty, nutritious treat. That’s a great idea, provided you obtain the worms from a reliable source so that you know the critters are disease free before offering them to your pets.

But what does it mean if you discover black worms living in your freshwater aquarium? Are the worms harmful to your fish? How did those pesky wrigglers get into your aquarium? How do you get rid of the worms?

Keep reading to learn everything you need to know about black worms in your fish tank and how to get rid of them.

What Are the Black Worms in My Fish Tank?

“Black worms” is the general umbrella term hobbyists use to describe the many species of tiny black-colored worms that can sometimes be seen swimming freely in the tank or emerging from the substrate.

Some black worms are harmless and make a good food source for your fish, while others can be parasitic pests that are potentially harmful to your pets.

Good Black Worms vs. Bad Black Worms

Generally, the free-swimming black worms you see cruising through the water column, crawling up the glass, or wriggling in the substrate are harmless to your fish. In fact, on the one occasion that I discovered detritus worms living in my fish tank, my goldfish made short work of them before I had a chance to take action to get rid of them!

The main problem with a population of black worms is that they don’t look good in your pristine fish tank. In addition, many worm species reproduce extremely quickly, so your aquarium could quickly become overrun by critters.

Identifying Species

Common black worm species that find their way into your fish tank include bloodworms, planaria, drain fly larvae, and blackworms.

All these species are different in appearance. For example, drain fly larvae are translucent and thin, whereas bloodworms are dark red with segmented bodies, and planaria are slender and flat with a distinct head.

Dangerous Worms

If you spot any worms protruding from your fish’s body, gills, or rear end, those critters are most likely to be parasites, such as flukes, that could ultimately harm or even kill your fish.

Rather than trying to pull the worms off your fish, we recommend placing the infected fish in a quarantine tank and treating them with over-the-counter medication.

Types of Worms

As previously mentioned, several black worm species could infect your fish tank. To treat the worms effectively, it’s important to identify the species correctly.

Drain Fly Larvae

Drain fly larvae appear as lively black worms you’ll see moving around in your fish tank, typically hanging out around your aquarium filter outlet.

These tiny creatures only measure between 4 and 10 mm long and feed mostly on leftover fish food and algae. Eventually, the larvae morph into tiny black flies that emerge from your tank when you remove the lid to clean it or feed your fish.

Drain fly larvae are often viewed as a food source by many fish species, so it’s likely your fish will quickly gobble up these black worms before their numbers have a chance to get out of control.


large amount of tiny bloodworms

Bloodworms are not actually black but dark red in color.

Bloodworms are commonly sold in fish stores and some pet shops as live food for fish. You can also buy freeze-dried and frozen blood worms, which make great occasional treats for your fish. However, bloodworms are very low in protein and contain a low range of amino acids, so they’re not suitable as a single food source for your fish.

Usually, any bloodworms in your tank will quickly be eaten by your fish, probably before you have a chance to remove them!


planaria worm on aqautic plant

Planaria worms are a well-known aquarium pest species. These black worms can reach a maximum length of 10 mm and have a distinctive triangular-shaped head, and they can be problematic if they become too numerous in your tank.

Planaria are flatworms found in both freshwater and saltwater tanks. These wrigglers are asexual, which means that the creatures can reproduce without the need for a mate. In fact, if a planaria worm is cut into two pieces, each part will grow into a brand-new worm!

Although planaria are not harmful to healthy adult fish, these worms are carnivorous scavengers, and they will eat fish and shrimp eggs.

Planaria usually get into your tank attached to plants, especially if taken from the wild environment.



Leeches are rarely seen in home fish tanks since the creatures live in wild habitats and are not sold as food for fish.

However, if you buy live plants for your tank, be sure to check the underside of the leaves to ensure leeches are not present before you add the plants to your aquarium. Although your fish will happily munch on leeches, these black wormlike creatures can be a pain to get rid of once they’re in your setup.

Mosquito Larvae

Mosquito Larvae

You can buy frozen mosquito larvae as food for your fish. In fact, betta fish eat mosquito larvae as part of their staple diet.

These little black worms measure around a quarter inch long and will eventually morph into mozzies, escaping out of the tank and into your home, providing your fish don’t eat them first!

Detritus Worms

Detritus worms are annelid worms that are often present in fish tanks, going unnoticed until their numbers increase. At that stage, you might only see a couple of detritus worms crawling on your glass, or you could find a large bunch of these critters creating a virtual carpet across the bottom of the tank.

Detritus worms are usually seen in a tank with gravel substrate, where they wriggle through the grains of gravel as they feed on organic waste and leftover fish food, rather like a kind of natural cleanup crew.

As their name suggests, detritus worms are attracted to waste matter in a dirty tank, and cleaning the tank regularly can prevent these creatures from becoming established.


Blackworms, or Lumbriculus variegatus, are also referred to as California blackworms. These creatures are found right across Europe and North America in shallow water bodies, such as swamps, marshes, and ponds.

The dark gray or black-colored worms can reach as much as 4 inches long and feed on microorganisms and organic detritus in the tank. The worms are easily identifiable from their large size and characteristic forked head section.

Blackworms can reproduce through a process called fragmentation, where the worm splits into sections, and each eventually becomes a new worm. In addition, the worms can reproduce sexually.

This type of black worm is a good find in your fish tank since these creatures make excellent food for your breeding fish and fry, being rich in proteins, vitamins, and fats.

Are Black Worms Good for Your Aquarium?

The presence of black worms in your fish tank is not necessarily a bad thing, depending on what species you have. In many cases, your fish will eat the worms to control the population, and the wriggling critters are harmless.

However, some worms will eat fish eggs, and the presence of other species is a sign that your tank is not as clean and well-maintained as it should be.

In addition, you want to enjoy your beautiful fish and your carefully chosen plants and decorations without the view being spoiled by a bunch of unsightly worms crawling all over your aquarium. So, from an aesthetic perspective, black worms are not a good thing for your fish tank.

How Can I Get Rid of Little Black Worms in My Fish Tank?

Getting rid of those annoying little black worms in your fish tank is not always straightforward, depending on the species.

Sometimes, eliminating black worms can be done by cleaning the tank and physically removing the worms. However, some worms can only be removed by using a chemical treatment.

Often, the medication used to kill black worms contains copper, which is dangerous to some fish species, invertebrates, shrimp, and snails. So, ideally, you need to get rid of the worms naturally and take effective steps to prevent them from coming back.

Clean Your Tank!

Most worms feed on detritus and leftover fish food within your tank. So, keeping the environment clean and tidy and improving the water quality is the best way to ensure these creatures don’t spread and become established.

Start by using an aquarium vacuum to remove muck and uneaten food from deep within the substrate, around plant bases, and under decorations. You’ll probably end up sucking out a few worms, too, which is a good thing.

As part of your cleanup, change around 25% of the water in your tank. Repeat the process the next day.

Clean your filtration system thoroughly, including washing the filter media in old tank water to remove any sludge that’s clogging the impeller and the media. Replace any spent filter media, refit the unit, and switch it back on.

Treat the Tank With Worm Medication

It might be necessary to treat your tank with worm medication, depending on what species of black worms you have in there.

If you’re unsure what drug to use, try to provide the experts at your local fish store with an accurate description of the worms, and they will be able to recommend a suitable medication for you to use.

How To Prevent Black Worms From Getting Into Your Tank

Of course, it’s better to keep black worms out of your fish tank altogether rather than having to get rid of them once they’re become established.

Fortunately, keeping worms at bay is pretty straightforward if you’re diligent in your tank maintenance and careful when introducing new fish and plants to your setup.

Maintain Your Tank Correctly

First and foremost, it’s crucial that you keep on top of your tank maintenance tasks, such as vacuuming the substrate to remove all organic waste material and carrying out partial water changes once a week. Trim away dead plant leaves and remove them from the environment, too.

Bottom line: If you keep your tank clean, tidy, and properly maintained, it’s unlikely that you’ll end up with a worm problem.

Don’t Overfeed Your Fish

Worms also enjoy munching on leftover fish food. After all, those fish flakes, pellets, and algae wafers are packed with delicious nutrition that the worms love.

Overfeeding your fish not only encourages worms to proliferate and thrive, but it leads to health problems for your fish and costs you money, too!

We recommend feeding your fish twice a day, offering only what the fish will eat in a couple of minutes. Take the time to remove any uneaten food immediately to keep the environment clean and tidy.

Avoid Overcrowding

The more fish you have, the more waste they produce, potentially increasing the work your fish tank filter system has to do to keep the tank clean and safe for the fish.

In an overcrowded tank, the bioload can become so great that the filter can’t cope with it, leading to unsanitary conditions that are perfect for worms. So, be careful that you don’t overstock your tank, and remember that the fish you buy from fish stores are usually juvenile specimens that will quickly grow!

Quarantine New Fish

Sometimes, worms and flukes can find their way into a healthy aquarium with new fish and other livestock.

When choosing fish, take your time and look to see that the fish are in good health without any obvious parasites, injuries, or other signs of poor health. When you get your fish home, remove them from their transport bag with a net, and place the fish in a quarantine tank. Observe the fish for at least a month before moving them to your main tank.

That should allow plenty of time for any diseases or parasites to appear so that you can treat the problem before moving the fish.

Check New Plants Carefully

Aquatic plants are also a prime way of introducing parasites into your fish tank. Often, tiny worms hide among the plant leaves and roots, sneaking into your aquarium undetected before spreading rapidly.

Always wash new plants under running water to flush away worms and other hitchhikers, and check the undersides of all the plant’s leaves to make sure there are no worms or snails lurking there.

It’s a good idea to use a worm treatment product solution to wash the plants before adding them to your main tank. That should kill off any parasites, leaving the plants safe to add to your aquarium.

Treat Imported Substrate With Worm Medication

Introducing some of the substrate from an existing aquarium to a new tank can be a good way of introducing beneficial bacteria to the habitat and speeding up the cycling process. However, that can also be a very good way of accidentally introducing worms and other parasites.

If you want to use old substrate, you’ll need to wash it thoroughly to flush out any worms that might be hiding there. You can also rinse the substrate in a worm medication solution as a further worm prevention method.

Final Thoughts

Did you enjoy our guide to black worms in your fish tank? If you found the article helpful, please share it with other readers.

There are many species of tiny worms that can find their way into your fish tank with new fish, attached to plants, or hiding in the substrate from an existing aquarium.

Most black worms are harmless to your fish, and many fish will even eat any worms they find. However, worms don’t look good crawling around your tank, and they are often indicative of a dirty environment, which isn’t good news for your fish.

You can get rid of black worms in your fish tank by vacuuming the substrate thoroughly and keeping on top of weekly aquarium maintenance and cleaning duties to prevent the worms from becoming established.

Did you discover black worms in your fish tank? What species were they, and how did you get rid of them? Tell us in the comments box below!

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