is your source for fish food, driftwood and all the essentials to keep your fish happy and healthy.


The worms!

(And other assorted small creatures.)


They're disgusting! They're everywhere! Will They kill my fish? I've got to get rid of them!

Don't I?

In this article, I thought I would review the interesting world of worms and other micro-fauna. Having so often heard of inexperienced fish keepers panicking at the sight of "something I didn't put there!", it seemed worthy of exploration. I plan on describing the most common species of worms as well as other small creatures that often turn up in aquaria. Hopefully this will help clear up any misconceptions and alleviate any anxieties on the subject.

*Photos on this page can be clicked for larger view.

"True" Worms Insect Larvae Crustaceans Others
Nematodes ("Roundworms", including microworms, vinegar eels) Bloodworms and Glassworms (Midge Larvae) Daphnia Hydra
Planaria (Turbellarians or "Flatworms" Mosquito larvae Ostracods (seed shrimp, clam shrimp) Baby snails, egg masses and snail trails.
Annelids (Earthworms, leeches, tubifex, blackworms, whiteworms) Dragonfly or damselfly larvae (nymphs) Cyclops *Important notes about aquarium microfauna.*
  Water beetle larvae Gammarus (scuds, sideswimmers)  
  Caddisfly larvae    




"True" Worms

The term worm is fairly broad and is used for many separate Phyla each with their own Characteristics. In fact some of the worms mentioned later don't even belong to this broad definition, but rather they are insect larvae or other types of animals.

Nematodes ("Roundworms", including microworms, vinegar eels)

Size - 8 meters(!) to microscopic. Those usually seen in aquaria are 2mm or less.

Coloration - Varies with species, but white species are the most common.

Defining Characteristics - Nematodes are very simple worms. Their shape is long and thin, like a piece of string or fiber. They have no body rings, setae (hair), eyes, or other externally visible features. Under a microscope, a stylet (needle-like structure) may be seen at the mouth end.

Movement - Most Nematodes seen in the aquarium live are benthic, living in the soil, or on plants, decor, etc. They can however swim erratically. The motion is often described as "S" shaped.

Food - Quite variable dependant on species. Again, most species observed by the casual aquarist feed on debris, bacteria, etc. within the soil or on surfaces.

Control - Proper tank maintenance (water changes, vacuuming the substrate, avoid excessive feeding) will keep the numbers down to unnoticeable levels. Copper treatments are effective, but should be used with caution. Nematodes are eagerly eaten by small fry and shrimp.

Notes - Most often, when an aquarist sees a Nematode, it is a simple scavenger, and of no harm to fish or plants. However, parasitic species exist. The general rule is if the fish and plants appear healthy, the worms are harmless.


Planaria (Turbellarians or "Flatworms")

Size - The most commonly seen smaller species are around 2 mm or less. A larger species is sometimes accidentally introduced, which can be around 1-3 cm.

Coloration - Somewhat variable, but usually described as brown or gray. They can appear almost white to black.

Defining Characteristics - Triangular shaped head on long, flat, ovalish body. Only some species carry the triangular head, and even of these, the head may not always appear triangular due to the fluid-like appearance of the worms' movements. Under magnification, eyespots and cilia are visible. When pestered, the worms can compress themselves into the shape of a tiny flat circle.

Movement - They travel smoothly along surfaces. Generally does not swim through water.

Food - Omnivore. Detritus, fish food, decaying plants or animals, etc. Not a hunter, but will eat eggs.

Introduction - With live foods, plants or objects collected from natural sources.

Control - Paradise fish, Bettas and blue gouramies are said to eat them. Place a small dish on the bottom of the tank. Place some frozen fish food, catfish tablets, or a small piece of meat on the plate. Cover with a container or dome to prevent any fish from stealing the food, but allow a tiny passage for the worms. Wait 12-24 hours and collect the worms. Copper treatments are effective, but should be used with caution. Proper tank maintenance (water changes, vacuuming the substrate, avoid excessive feeding) will keep the numbers down to unnoticeable levels.

Notes - Planaria are generally harmless except to eggs and possibly newly hatched sessile fry. The main annoyance stems from their unsightliness, as well as their ability to over populate a tank. When over-abundant, they can affect the biological cycle of the tank, and if a massive die-off occurs, the decaying worms could have a negative effect on the system. In numbers, they can also compete for food with more valuable tank mates, such as ground feeding fish and snails.

For those who don't mind their appearance and are not breeding fish, the Planaria population can be minimized by ensuring not to overfeed the fish, and by providing adequate aquarium maintenance routine which includes cleaning the filters and vacuuming the gravel.

Though the name "Planaria" is still very commonly used, it is no longer scientifically valid. "Turbellarian" is the proper name for these species.

This is the most common Planaria seen in tanks, being barely visible (2mm). Note the two eyes (dark spots) just above the brain (bigger brownish mass).

A larger species of Planaria (1 cm) very commonly found in local water sources. Sometimes inadvertantly introduced on plants and decor from unhygenic sources.


Annelids (Earthworms, leeches, tubifex, blackworms, whiteworms)

Size - Microscopic to over 1 foot long.

Coloration - Usually pinkish tan to red, but can vary among species. Yellowish and green are also common.

Defining Characteristics - Any worm with a long cylindrical body that is 'cut' by transverse rings is an Annelid. Microscopically, hairs can be seen, growing seperately from each body section.

Movement - Terrestrial Annelid worms are usually seen to move through contractions and expansions of their bodies. They have little strength to lift themselves off the ground. Aquatic Annelids use the same mechanisms for movement, but the effect is less pronounced. Extremities are often lifted off the ground. Aquatic species are more skilled at moving through their medium, whereas terrestrial species often roll around and writhe in confusion when placed in water.

Food - Detritus.

Control - Only manual removal or being eaten by a fish can be recommended.

Notes - Unless deliberately added to an aquarium, large Annelids such as earthworms are rarely found in aquaria. Aquatic species can survive for quite some time in the gravel. Terrestrial species usually die within a day. Even a single larger dead worm can cause severe disruption to the water quality of the tank. Unless eaten, they should be immediately removed before they die or burrow into the ground. Their use as a fish food is acceptable as long the worms do not manage to escape into the substrate.

Species smaller than one millimeter are common, though shouldn't be seen in excessive numbers in the open or on the glass. Treat over population as for Planaria above.

An aquatic Annelid found in a vernal pool. Approximately 2.5 cm long. Another image of wild caught aquatic Annelids. These measured about 0.5 cm. Use of wild collected plants or decor often introduces such species into the tank.  
The tunnels are the homes of tubifex worms, a commonly used live food. They also naturally occur in wild sources. The tubes are built by the worms out of surrounding debris, and cemented with excretions they produce.
This unidentified Annelid was found in the filter gunge from a power filter.
This unknown worm, when seen directly with the eyes, appears just like a small Planaria. Magnification through a microscope was necessary to distinguish it from a Planaria.



Insect Larvae

Many species of insects spend part or all of their lives in water. Here are 3 familiar kinds of aquatic insect larvae. All become free flying insects.

Bloodworms and Glassworms (Midge Larvae)

Size - 1 mm to a possible 25 mm when full grown.

Coloration - Red. Rarely black, brown, green, or transparent for glassworms.

Defining Characteristics - Tend to build tunnels/trails within the sediment. They sometimes leave them to search for food.

Movement - Wriggly side-to-side (figure eight style) motion when swimming. May also crawl or hang on surfaces while feeding.

Introduction - Used as a live found. Can sometimes be introduced with other live food, especially if collected in the wild. Also may come with plants or objects collected from the wild.

Control - Usually not necessary as they are greedily eaten by fish. Care must be taken when using them as food. Those that escape tend to bury themselves in the gravel, often dying there. In excess, this can pollute the tank.


Mosquito larvae

Size - 0.5-6mm, depending on age and species.

Coloration - Pale brown to black.

Defining Characteristics - A breathing tube is located at the base of the abdomen. They must return to the surface every few minutes to breathe. They may be seen resting upside down at the surface with their breathing tube just poking the water's surface.

Movement - When swimming through the water, they move in quick, repetitive jerks. Either they jerk their heads to their tails and back again, or they switch their head and tail positions quickly but continuously. The pupae swim in quick hop-like motions. When feeding they graze on surfaces.

Food - Algae, detritus, possibly tiny micro organisms such as plankton, bacteria, etc.

Control - Mosquito larvae can easily be caught at the surface with a net. They are also eagerly eaten by fish.

Notes - Mosquito larvae are harmless to aquarium residents, and are an excellent fish food. However, if no fish are present to eat them, they will eventually emerge as flying, stinging adults. The growth rate depends on temperature. (Higher temperatures, quicker growth) It can take as little as one week for mosquito larvae to become adults. The pupation period is 1-3 days.

Larvae feeding on decaying plant matter. Mosquito larvae are excellent algea eaters, quickly cleaning any holding vessels. Somehow, they even manage to deal with 'green water'. If not for the biting adults, they would have long ago found a place in the hobby for this function. The black-headed larvae is a different species of mosquito. As is true for all other creatures mentioned on this page, there are various species for each type of shown.
As seen in this picture, unhindered, mosquito larvae can be quite numerous indeed. Top view of a group of larvae.
Close-up of common mosquito larvae.

Another close-up, with pre-adult nymph in the center of the picture.


Dragonfly or damselfly larvae (nymphs)


Size - 0.5 mm - 20 cm depending on age and species.

Coloration - Variable. Dark brown, gray, white, or black are the most common colours..

Defining Characteristics - Noticeably big eyes. They walk around the bottom of the tank using their long legs. They often hold their abdomens horizontally or diagonally off the ground. There are no gills along the entire edge of the abdomen as seen in mayfly larvae. Damselfly Nymphs breathe through gills that extend from the end of the abdomen, which look thin and spike-like, or feathery. Dragonfly nymphs also breathe through small extensions at the end of the abdomen, but these are short and dificult to see.

Movement - See "Defining Characteristics" above. They are also capable of climbing decorations very well. When walking, they tend to move slowly. If they move quickly, they tend to seem jerky in appearance.

Food - Exclusively predatory. Will eat anything that moves that it can catch, which includes fish its own size!

Control - Will usually be eaten by any fish that can swallow it. Can easily be removed with a net or container. Proper aquarium maintenance and filtration will usually (but not always), hinder its survival. Discontinue use of live food until removed.

Notes - Dragonfly and damselfly nymphs are rarely introduced into aquaria, but it can happen. They should be removed quickly as they are efficient hunters and greedy eaters.

Close-up of larva's head. Note the armlike mandibles, used for snatching prey. These can extend amazingly far.
Close-up of dragonfly larva.

Newly emerged larva using his shed skin for support. All aquatic insect larvae go through several moults before emerging as adults.



Mayfly larvae (nymphs)


Size - 0.5 mm - 10 cm depending on age and species.

Coloration - Variable, though usually pale. Brown, gray, or yellow are the most common colours..

Defining Characteristics - Three 'tails'. They walk on surfaces of the tank using their long legs. Nymphs breathe through feathery gills which are located along the length of their abdomen.

Movement - See "Defining Characteristics" above. They are also capable of climbing decorations very well. Their movement when walking is smooth, almost gliding in appearance.

Food - Dependant on species. Most are herbivorous, while some are detrivores or omnivores. Very few are carnivorous, and those that are usually eat worms and other insect larvae.

Control - Will usually be eaten by any fish that can swallow it. Can easily be removed with a net or container. Proper aquarium maintenance and filtration will usually (but not always), hinder its survival.

Notes - Mayfly larvae are rarely introduced into aquaria, but it can happen. Most species are fairly benign. While introducing them on purpose is not recommended since they could bring in disease or parasites, if you should happen on one in your tank, it can usually be safely left there to finish it's growth. That is, if it can survive the fishes' appetites!

Larval mayfly. Sometimes called a water penny. Notice the very prominent gills running along the side of the body.




Water Beetle Larvae (Water Tigers)


Size - 3 mm to 4.5 cm..

Coloration - Usually dark brown or black. Other colours occur less often.

Defining Characteristics - The mandibles (jaws) are quite unique to these species. They are noticeable large, and generally kept in an open position. Under a microsope it can be seen that the tips are hollow, to allow the larva to suck the juices from it's prey without any escaping into the surrounding water.

Movement - Generally a slow mover with a clumsy appearance. Can crawl surfaces or debris, wriggle on the ground, float or sink at will, and hang at the surface of the water.

Food - Extreme carnivore!Will catch and consume prey (including fish) even much larger than themselves! May eat algae and detritus on occasion, but this is only a supplement. Plenty of meat is a necessity for these animals.

Control - Much larger fish such as Cichlids over 15 cm will usually eat these larvae with relish. Otherwise, manual removal must be performed, and quickly.

Notes - The larvae of aquatic beetles usually breathe through their skin. Some species however sometimes have 2 spiracle at the tip of their abdomen to gain Oxygen from above the surface of the water.

Most adult beetles from this group are also highly predatory.

This 'water tiger' clearly shows the fierce mandibles that give it its well deserved reputation. Many species have the well developped  pseudo-legs along the abdomen, as this one shows, but they can be reduced or even absent in many other species.



Caddisfly Larvae


Size - up to 20 mm.

Coloration - Variable.

Defining Characteristics - All caddisfly larvae live inside a case they fabricate themselves. The appearance of the case and materials used are dependant on species, though if appropriate materials are not obtainable, the larvae will usually make due with what is on hand.

Movement - Usually these larvae crawl on the ground or over plants and rocks, searching for food. However, many species can swim quite smoothly and quickly, bringing along their cases wherever they go.

Food - Dependant on species. In the author's experience most seem to be herbivores or detrivores. Some carnivorous species do exist however.

Control - Some fish or other aquarium denizens such as shrimp can sometimes pulls the larvae out of its case and eat it. But usually caddisly larvae avoid predation effectively by withdrawing into their cases. Manual removal is the only effective control.

Notes - Caddisfly larvae use an orally produced silk to bind the materials of their case together. The cases are extremely hard and durable as a result.

The larvae can sometimes be seen bouncing their cases up and down. This is a technique used to enrich their Oxygen supply, which is breathed in through gills located at the tip of their abdomens.

Caddisfly larvae are not dangerous to fish, but may become a problem with the plants if too many are present. Even so, their growth period in the water is long, and and as such, their appetites are not overly ambitious. If discovered, they can usually be left without significant problems.





Though in no way can any of the following creatures be mistaken as worms, they are very common, and can be mistaken as harmful pests in the aquarium. Some species are extremely small, and may need a microscope or strong magnifying glass to be identified.


Size - 0.5-3 mm.

Coloration - Usually red, but also green, beigish or clear.

Defining Characteristics - Daphnia look like fleas.

Movement - Using their arms, they swim through water in a vertical hopping or bouncing motion. They rarely rest. They are not grazers. Arms do not retract.

Food - Suspended algae, bacteria and yeast.

Control - Almost all fish will eat them.

Notes - There are several species of Daphnia, Daphnia pullex, Daphnia magna, and Moina spp. being most common in North America.

Reproduction is through parthenogenesis (producing live young, which are clones of the parent) throughout most of the year. In the fall, males are produced, and sexual reproduction occurs. Eggs are later deposited. Diapause also occurs, with eggs or young surviving within the parent until spring, when they hatch or emerge.

The author can think of no situation where Daphnia could directly cause problems. They do not eat eggs, fry or plants (other than suspended algae), and are beneficial in several ways. They are an excellent fish food, and can control 'green water'. In theory though, they could pose a threat to extremely small fish fry.

The creature to the left of the picture is a Bosmina, a smaller relative of Daphnia.


Ostracods (seed shrimp, clam shrimp)

Size - In North America, almost microscopic to 2 mm. Larger species exist.

Coloration - Variable depending on species. White, gray, black, orange, green, and mottled have been seen by the author.

Defining Characteristics - Virtually always kidney bean shaped, though this may be difficult to see in smaller species. Their legs can be retracted into their shells.

Movement - When swimming, their motion is smooth and rapid through the water. They also swim along surfaces searching for food.

Food - Fairly omnivorous. Detritus, fish food, decaying plants and animals, even bacteria and other tiny micro organisms. Also algae, but usually not full sized plants.

Control - Complete eradication is often unsuccessful, except for larger species, which seem more fragile. Regular aquarium maintenance and proper filtration usually keep the populations down to unapparent levels.

Because of their structure, Ostracods are extremely resilient against toxins. By closing the two shells, they can survive extended amounts of time in the presence of medications and pesticides. Even if they die, the shells serve to protect the unborn young until conditions are right again.

Notes - Ostracods are called seed shrimp or clam shrimp, and with good reason. The creatures exist inside opposing shells that look and function just like those of clams. Dead Ostracods are often wrongly identified as clams.

In most species, no males have ever been observed. Reproduction is by parthenogenesis, the production of clonal young. Many species also experience a yearly diapause. The adults die when their habitat dries up, or winter approaches. The young are safely harbored within the body of the parent, and emerge the following spring or rainy season.

Ostracods could only become a problem if they extremely over populate a tank, a condition the author has never seen in a fish aquarium. Only in infusoria jars does this seem to occur.

Because of their hard carapace, Ostracods are usually unappreciated as fish food. Their use as such is not recommended.

This Otracod was about 1/2 mm in size. Note the 'bean' shape, which is common in this group.
This view shows the legs of an Ostracod, which are often hidden within the carapace.

This photo was obtained using the super-macro setting of a digital camera. These small specimens were barely in the range of what the camera could capture.



Size - Up to 1 mm. Specimens caught outdoors in spring are significantly larger.

Coloration - White to tan.

Defining Characteristics - Females carry 2 egg pouches on their tail.

Movement - Travels in quick jerky hops, along surfaces or in the open water.

Food - Omnivore. Will eat detritus and algae, but will also hunt smaller organisms, such as bacteria, protists, worms, etc. Will attack tiny fish fry, but not larger ones such as guppies or Cichlid fry.

Control - Complete eradication is all but impossible. Proper aquarium maintenance and filtration will keep their numbers down enough to be unapparent. Providing a good current, even without filtration, will also keep the Cyclops in check.

Notes - Cyclops are generally harmless to all but the tiniest of fish fry. (Guppy and Cichlid fry need not worry.) In fact, Cyclops are an excellent "first food" for those young fish capable of catching them and consuming them.

As long as the tank is well maintained, Cyclops pose no danger. However, the author has had one negative experience keeping a Betta in a 5 gallon unfiltered tank (and hence no current.) The Cyclops overpopulated to such a point that they regularly attached to the Betta. Symptoms included sudden spasms and flashing. When a bright light was directed at the tank, the water appeared clouded with Cyclops. The addition of a small box filter took care of the problem.

A Cyclops female carrying egg cases.


Gammarus (scuds, sideswimmers)

Size - 1/2" when mature.

Coloration - Grey, brownish, or greenish.

Defining Characteristics - These shrimp keep their bodies curled when resting or eating. In this position, 2 of their legs can be seen reaching upwards beyond their carapace. When swimming, more often than not, they swim sideways. They prefer to roam on or under substrate, or hide under the gravel, decorations, etc.

Movement - See "Defining Characteristics" above.

Food - Plant debris, possibly tiny alga and microscopic organisms.

Control - Eagerly eaten by fish. Usually cannot long survive in a typical aquarium due to high temperatures, lack of hiding spaces and food.

Notes - Scuds are fairly harmless themselves, perhaps only being a threat to very small sessile fry and eggs. Even then, there are usually not enough of them to pose a real problem. While they are considered a nutritious food for fish, wild caught specimens should not be used as they can possibly transmit serious diseases to the fish.





Size - Up to an inch when full grown.

Coloration - Usually green.

Defining Characteristics - The Hydra is single animal with several tentacles fanning out from a central stalk, which remains fixed to one location. Individual Hydra produce 'buds'.

Movement - Beyond the movement of the tentacles, which quickly retract if food is caught, the main stalk appears stationary. However, over the course of a day or two, Hydra may manage to move up to a centimeter or more from their original location. A Hydra can also detach itself from its attachment point and fall, or be carried by the current. When disturbed, the whole creature can compress into a round, jelly-like green spot.

Food - Requires live food. Small fish fry can be taken. Any smaller live fish foods will be consumed, such as Daphnia, Cyclops, small worms, etc. Even if visible food is not available, Hydra can efficiently sustain itself by consuming almost invisible, but ever present, micro fauna, which always exists in established tanks.

Control - Paradise fish are said to eat them.

Aquatic medications containing Copper will kill Hydra, but are recommended as a last resort due to risk of poisoning fish and plants.

I have seen the following treatment posted in several places on the web. Some swear by it, others consider it dangerous for the fish. (deaths reported) Use at your own risk!

Attach a wire to each pole of a 9 volt battery. Place the ends of the wires into the tank water, as far apart as possible. If the setup is working correctly, a fine stream of bubbles should be seen from one of the wires. The Hydra will start falling after about 20 minutes. The treatment should go no longer than 3 hours, keeping an eye on conditions the whole time. A daily 50% water change for 3 days is recommended since Copper leaches into the tank via one of the wires.

Notes - Hydra are polyps that have never been observed to change into jellyfish, which is in fact, the Order they belong to. Reproduction occurs through budding, with one Hydra producing one or more young at a time as off-shoots from their bodies.

A hydra, in normal relaxed (extended) position.
A Hydra which has been disturbed will usually retract itself, appearing in extreme cases as a round blob, without the arms unfurled.

A Hydra, as seen through the naked eye. Note the clonal bud in formation.


Baby snails, egg masses and snail trails.

Mentioned because they each can cause confusion.

Baby snails can be almost microscopic for some species, and at that size, are generally unrecognizable as being snails. Their extremely slow but steady movement along the glass can be a key in identifying them. They will often seem to be more densely present in some areas, and some species seem to prefer remaining near the upper third of the tank.

Snail egg masses first appear as a blob of clear jelly, and can range in size from several millimeters to several inches long or wide, dependant on species. Within a few days, whitish or yellowish spots will appear, which grow ever bigger as time passes. The spots of course are the developing young snails within their eggs. After a few days or weeks, the baby snails will hatch and wander away, leaving the blob clear again. The jelly will drop and decay within a few days. Most species of snail do lay eggs, but a few, like the Asian trumpet snail, produce live young.

Among the cutest mistakes made by a newbie (including the author when she was but a wee lass, new to the hobby) is mistaking a snail trail for something else. Snail trails are usually noticed in the morning, having seemingly been produced overnight. For some reason, even when a snail is being actively watched by day, they do not seem to leave the deep gouges in the algae that are produced when the lights are off. They must graze more efficiently at night!(g)

Snail trails are usually seen through the algae on the glass. The furrow is usually 1-2 mm wide, and continues erratically along the glass. Less often, trails can be seen on algal mats on decorations. Amazingly, some smaller species of snails can create trails as wide as those produced by larger ramshorns and apple snails.

Control - Snails are often kept as interesting inhabitants in their own right. But when the population explodes, many consider them a visual nuisance, and in numbers, they can upset the biological balance of the tank. Some species can also harm plants. Occasionally, snails (especially wild caught) can harbors parasites harmful to fish.

Snails can be controlled by manually collecting them, or through the use of snail killing preparations. Be aware that the use of such products is not without risk to your plants and fish, and should really only be used as a last resort.

Tasty foods such as zucchini, cucumber, fresh fish or pellets, can be used to attract the snails for manual removal. Simply place a piece in the tank and leave overnight. (The food may need to be covered loosely with a small plate to keep the fish from reaching it.) Collect in the morning and dispose. The procedure can be repeated every 2 nights if necessary, but take care the water does not become fouled..

Keeping the tank clean, free of debris and detritus, minimizing algae, and reducing the amount of food fed to fish are all recognized methods for keeping snail populations down. Quarantining new plants and sterilizing decor objects will prevent their introduction into the tank, as will collecting/buying live food from snail free sites. Soft water and low pH are also ways that can be used to eradicate many species. The lack of minerals in the water can actually 'erode' their shells, killing them, but this can take weeks or months to work.

Close-up of a mass of snail eggs. The embryos are almost ready to hatch.

Another photo of a snail egg mass. These have been only recently deposited, and the embryos have only just become apparent.

Note the mouth of this snail, which contains raspers for ripping food and algae off surfaces.







Some things you should be aware of.


But even with these tips, unwanted introductions can still occur. Being aware and prepared for the possibility is your greatest asset.



Home Page Are Your Fish Really Suffering From Disease? The Worms! A Cheapskates Guide To The Aquarium Hobby
Copyright to author. Email for questions. Comments eagerly invited.
Last Updated: July 31, 2008