Worms in the aquarium

African cichlid in fish tank

Worms in the aquarium. Are they common? Who can get them? What can I do about them? And what do they mean for the health of my fish?

Worms are nocturnal creatures. If you can’t see them during the day it doesn’t mean they’re not present. Because they are light-sensitive, they remain in the darkness of the gravel, rocks, plants or any hideout available during the day or lighting hours.

Obviously the best way to detect any nocturnal creatures is therefore only at night or when the tank is dark.

It may take several hours after the lights have been turned off and the room darkened before the creatures appear. Using red light will make them visible for the human eye, but will not be recognized by the worms. Rapid movements should be avoided it will scare the worms back into hiding.

Worms grow at a fast rate; the same applies to their reproduction. Once worms have been spotted, and action has been taken to eliminate them, it is this fast rate of growth and reproduction that can be misleading if their successful removal from the tank is believed.

Clear cloudy aquarium water, remove nitrates and simplify aquarium maintenance

Worms eat everything they can find in the aquarium, which initially makes them appear to be ideal bottom cleaners. Nevertheless the fast growing population makes it a priority to remove all the worms that can be found. Some initially small and harmless looking species can grow to a solid 24-inch sea monster. No matter how long the species will get, there is usually more worm than meets the eye.

Worms can be introduced to the aquarium in various ways, through plants, new fish, fish food, snails etc.

One of the biggest threats for reef and marine set ups are the bristle worms. A fast growing population does not only do damage, but can also lead to attacks on crustaceans, corals and anemones. Bristle worms are visible and can be identified by the many bristles. These worms can grow into 24-inch creatures.

Next to fairly visible worms that are considered non-parasitic, there are of course external and internal parasitic worms that will harm the fish.

Parasitic worms are the roundworm, tapeworm, thorny headed worms, and flukes.

One rule applies, to calm the nerves of the hobbyist; most worms seen outside the fish are usually not internal parasites and not considered as harmful to the fish. Parasitic or harmful worms will not be visible on the gravel or the glass of the aquarium.

Noticing small white worms the size of a few millimeters crawling along the glass are either flat or roundworms. Flat appearances are planaria (flatworms) and the more thin and wiggly ones are nematodes (roundworms). Both of them do not exceed 4-5 millimeters. Both types are harmless to fish and thrive on excessive waste in the water column.

To free the tank of these worms simply do not feed your fish for 3-4 days as they will often feed on them.

The fluke is a parasite worm that is visible on the skin or gills of the fish. Measuring only 0.3 – 0.5 millimeters in length, this oval shaped worm can spread and often result in fish death.

Internal parasitic worms can not easily be diagnosed. Advanced stages can be visible by ulcers on the skin. On the other hand, not all internal worms do pose a threat to the fish.

First signs of internal worms are an increased appetite without weight gain or even weight loss while consuming increased amounts of food.

The tapeworm is the best known though not very common.

Worms of Concern

Anchor Worms

Young anchor worms are free swimming crustaceans that bury themselves into the fish’s skin. It takes several months before the worm becomes visible in form of holes or ulcers on the fish’s body. After laying eggs, the worm dies off.

Since the worm can not be removed by hand, a potassium permanganate bath for about 20 minutes should cure it (dosage 10ml/l).

Thorny Headed Worms

Visible symptoms are white or green threads on the gills. The fish often scratches on objects in the aquarium.

The thorny headed worm is similar to the anchor worm, only smaller in size. It attaches itself to the gills. The cure is also a potassium permanganate bath for 20 minutes (dosage 10ml/l).


The symptoms are mucus covered gills and/or body, red spots on the skin, fins appear eaten away, as well as rapid breathing.

Flukes are flatworms and are similar in appearance as Ick and can be better viewed with detail through a magnifying glass. Flukes will destroy the gills and kill the fish if left untreated.

The best cure is a potassium permanganate bath for 20 minutes (dosage 10ml/l).

Thread Worms

Thread worms are internal fish that sometimes emerge from the fish’s anus. This parasitic infestation can be fatal if not treated in time. Preferred treatment is parachlorometaxylenol soaked fish food and a bath in the same for several days (dosage 10ml/ liter).


These external parasites are visible on the skin, gills and fins of the fish and are similar in appearance to Ick.

Since they attach themselves to the fish, the best method of removal is a bath in a salt solution for 20 minutes (dosage 2.5 % salt to water). During the bath, most of the leeches will simply fall off; the ones remaining can be removed with a pair of tweezers.

Copper sulfate has been used to successfully remove and control less harmful worms. Over time, copper has been found to do more damage than good concerning the overall balance of the aquarium. The side effects of copper are rarely in relation with the possible benefits.

How to build a worm trap

Next to commercially available worm traps, it is fairly easy to make one at home. All that is needed is a plastic container or jar with a lid.

Using a razor blade or sharp knife, the lid is cut in X shape. The corners are then pushed slightly inward to form an opening in the lid. The size of the opening varies, depending on the size of the creatures to be trapped.

For the trap to work properly it is important that the worms do not see the “bait” but rather smell it. The container should therefore not be transparent.

For bait, clam and shrimp meat can be used as well as any fish meat available. The bait should be prepared in a way that is small enough for the worms but just short of being mashed up totally.

The container is then placed in the area where the worms are suspected and kept there over night. Adjustments to the size of the lid opening and the bait source can be made for optimum results.

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49 thoughts on “Worms in the aquarium

  1. Ashanti says:

    okay so i just got my fish today and everything i put in the 3.5 gallon tank is new but my sister and i went down to the creek by my house and got some rocks so we took some of them home and washed them with a tooth brush and warm water and then we set up the tank and just now i was looking at the tank and i see a small thin kinda white worm just swimming around so i freak and i get some water from the tank and i but my better fish in it and now i don’t know what to do plz help

      • Ashanti says:

        the worm is about a cm long maybe smaller i don’t know its whiteish and like its body goes up and down when it swims is it harmful to my fish? how can i get rid of them??

    • Thilo
      Thilo says:

      Okay, your tank is brand new, so there is no issue to remove everything and fill it back up with water. It is likely that the worm hijacked its way to the aquarium on one of the stones you collected. Generally, it helps to keep these stones soaked for a while in a separate container before adding them as decor. Please feel free to read about new setups and cycling an aquarium on this site.

  2. Wendy Young says:

    I have a cichlid that seems to have red worm like things on its gills. Resembles anemone more then anything. Also has ich treated with kordon rapid cure. Malachite green main ingredient. Seems to have cleared one side but still on the other. Should I isolate the fish? Is there a different solution I should try?

    • Thilo
      Thilo says:

      It definitely is parasitic and it will impact the other fish. You can try a potassium permanganate bath which will cure the problem. Do keep an eye out for the other fish to see if they show symptoms. Malachite green works as well, especially if you decide to treat the entire aquarium. A good maintenance session in-between treatments might be advisable.It will help in the long run, keeping in mind that fluke eggs hatch within 3 weeks, so the treatment needs to be extended accordingly. Make sure the tank is well aerated during the period of treatment.

  3. Saniya says:

    My fish give birth to baby fishes and then suddenly i saw many red worms inside my tank..how come they get inside it and what should i do to get rid of them and are they harmful for my fishes??

    • Thilo
      Thilo says:

      If you can see the worms and if the worms are not attached to a fish, they are non parasitic and quite harmless. The worms you see thrive on water with high nutrients. In most cases you have worms in the tank without knowing. The numbers are small and you usually do not detect them until they start reproducing if the water becomes high in nutrients.
      To solve the problem, reduce the fish food significantly and feed only every other day. Most fish eat these worms. Increase, or adjust your maintenance routine and or use bait (zucchini on a fork) to attract and remove the worms.

  4. Sarah says:

    Hi I just noticed these really tiny gray worms with a little black at the end type worms in my turtles tank. There probably are hundreds of them! Should I remove him? Will they harm him? How do I get rid of these?!

    • Thilo
      Thilo says:

      Detrius worms cause no harm to a turtle. These worms are actually quite common in turtle tanks because of the amount of food and amount of waste created in consequence. You should start vacuuming the gravel, which will remove most of the waste. Fish do no tolerate chlorine, but turtles can tolerate tap water treated with chlorine. This is a quick way to rid the worms as they do not take chlorine well at all.

  5. Mauro II says:

    There is a white very very tiny like worm wiggling in my aquarium. All my goldfish died because of Whitre Spot, and I treat them and it goes away but after a day they got fin rot instead. There is a dead fish day by day. Is the tiny worm harmful when I put new fish? your answers will be a big help for the crisis I am facing right now.

    • Thilo
      Thilo says:

      “White Spot” disease is generally known as Ick and is a parasitic disease. The life stages of this parasite spans about one week. The stages include a feeding and reproductive stage. You can read all about the details right here, just tape Ick into the search box on the top right of this page. You need to eradicate the parasite and treat your aquarium with medication immediately or you will lose all your fish.

  6. Albert says:

    I have about a billion very very tiny little white worms all over my tank floting around in the water and on everything. What are they and how do i get rid of them?

    • Thilo
      Thilo says:

      Yes, these are detrius worms. Please see detailed answer just below. Improving water quality is the key. These worms are harmless, but a sign for a less than optimal aquatic environment.

  7. Veronica says:

    I have no fishes in my tank for a while but there is still water, some tiny brown things swim around it and I put nothing in the tank, they’re as small as the top of a pencil led what are they ?

    • Thilo
      Thilo says:

      These are likely detritus worms, harmless and actually beneficial in reasonable numbers. These worms help break down waste (same as earthworms). They populate the gravel and are rarely seen outside their habitat. Reasons for them to wash out into the water column is explosive growths due to accumulating waste/ detritus or the lack of oxygen, both are somewhat related obviously.
      Fish will eat these worms and an increase in maintenance will control the worms without issues.

  8. Lochlan.Whaites says:

    My comet has 2 to 3 white worm like objects coming from the inside of the gill (1 gill) that go outside and back in when the fish breathes. The surronding area is red and looks inflamed. The gill that has the worm like objects is bigger than the other gill and unable to close fully.
    It rubs this gill on the side of the fish tank.
    No new fish have been introduced for 8 months and the food hasn’t changed in 2 months.

    • thilo
      thilo says:

      It definitely could be a parasite and should be treated. We do not diagnose via email, but it could be either gill flukes, anchor or thorny headed worms. In either case, and in general, a potassium permanganate (10ml per liter) for 20 minutes will provide a likely solution. Keep a close eye on the progress, but especially the other fish, anchor of fluke worms can show on other parts of the fish. Look for red points and ulcers.

  9. Jamie says:

    I had some gravel that was in abother tank for about 1 and a half months and it had maggots what should I do get new gravel or clean it

    • Scott @ Algone
      Scott @ Algone says:

      You can rinse it thoroughly and then boil it. That should take care of any potential residuals. Just don’t use any soaps or cleaners, fresh water only.

  10. Madison Andrews says:

    Hi i have a betta fish and i had to take him out of his tank because he has fin rot… Well, i can now see many tiny little brown worms swimming around in the vacant tank. These worms are about as big around as a hair and about a centimeter in lenght. Do you know what they are and if so are they dangerous?

    • Thilo @ Algone
      Thilo @ Algone says:

      The worms are completely harmless, but indicate a few issues with one or more of the following:
      high bio load/ too much food/ insufficient filtration/ lack of filter maintenance/ general maintenance (gravel vacuum) water changes

      These worms strive on high amounts of decomposing organics (mulm) the more detritus the more worms there will be. If you correct and improve the water quality, the worms will disappear. Water quality is also the main trigger for fin rot.

    • Scott @ Algone
      Scott @ Algone says:

      Take pictures of the fish or describe the condition as good as you can to your local aquarium store. They should be able to recommend and sell you the correct medication in order to treat the fish. Just make sure you pick a store with knowledgeable staff.

  11. Margaret Karinen says:

    HI, I have Garra rufa. A few days after I got them, some of them died and had these holes in their stomachs or close to the stomach area. I also noticed some little white worms on the bottom of the tank. It looked to me like the worms came out of pods. I am not sure. For about 5 days no more fish died, then yesterday 4 more died and I notice the hole in them. Does this sound like the ‘Pod worms’ are killing my fish? if so, is there something to deworm them? Thank you

    • Thilo @ Algone
      Thilo @ Algone says:

      It is quite likely that the worms are parasitic. You will need to treat the fish. There are several products available such as “Parasite Guard”. Follow the instructions and repeat treatment after about 2 weeks as some parasites have longer life cycles.

  12. Phillip baltzer says:

    I noticed little brown worms the size of pencil lead tips on the inside glass of one of my aquariums. is this dangerous to fish and how do I get rid of them

    • Thilo @ Algone
      Thilo @ Algone says:

      Out of the most common worms, roundworms (long and thin) are mainly whitish, annelids (cylindrical body) pinkish red, and flatworms (triangular head/ flat body) best described as mostly brown/ gray in majority. Best guess, these are detritus worms.
      Detritus worms are not harmful to fish and their food source consists of united fish food, decaying organic materials.

      These worms are best collected using a worm trap. You should also adjust your aquarium maintenance schedule if the worm issues persist. Avoid overfeeding and detritus in the gravel.

  13. Carmen says:

    Hi I recently found the free swimming white worms in my tank and I am a beginner fish keeper and I freaked out when I saw them. I have one male betta fish in my tank and it seems like he likes snacking on them.
    However what i am concerned with is whether it is harmful to humans? I did some reading and it said that these worms are nematodes or roundworms? I’m just super worried because sometimes when i clean the tank, i spill or sprinkle a bit of water on my desk (my tank is on my desk) and i just wipe it away with tissue and sometimes I wipe my eyes with the back of my palm after cleaning the tank (without washing hands yet).

    • Scott @ Algone
      Scott @ Algone says:

      Hello Carmen, sensible precautions are always recommended. While there isn’t generally a problem, it’s always a good idea to wear rubber gloves when reaching into the fish tank and thoroughly washing your hands when aquarium maintenance is complete. If you spill some water be sure to wipe it up immediately and use soap and a disinfectant if you are concerned about any parasites. Just take sure you don’t contaminate the aquarium with any soaps or disinfectants. I wouldn’t be too alarmed, but at the same why not avoid the risk of any contamination by taking appropriate precautions, no matter how remote the possibility.

    • Scott @ Algone
      Scott @ Algone says:

      These worms/parasites are generally harmless to humans. Nevertheless it’s always a good idea to wash your hands after reaching into the aquarium. You could also use long sleeved rubber gloves.

  14. Brittany says:

    I have a 30 gallon tank with 2 snails and a 3 inch suckerfish along with a few other tiny fish. The tank was set up 2 days ago with clean water new gravel and now I’m seeing clear/ white worms that range from centimeter long to about 3 inches and are rapidly producing. What might they be and how can I get rid of them? The longest one I’ve seen is about 3 inches and has attached itself to my baby suckerfish.

    • Thilo @ Algone
      Thilo @ Algone says:

      With the tank being only 2 days old, the worms might have been imported by either the, decor, substrate, plants etc. In this case I would recommend starting over making sure the setup is free of worms from the beginning. Please observe your fish closely and make sure they are not harboring worms. You might have to treat the fish first if you notice any signs. Generally, if you see the worm outside, they are not likely harmful.

  15. Krystle says:

    I have had tiny white/clear worms on the glass if my freshwater tank for a few months now. I have done vacuuming and water changes and treated my tank with copper power green which was recommended to me by a pet store. Nothing has helped and they keep coming back. How do I get rid of them? Is there something I can treat my tank with? I believe they are either planaria or detritus worms I’m not 100% sure.

    • Thilo @ Algone
      Thilo @ Algone says:


      Copper can easily be overdosed causing issues that massively outweigh any benefits. It can hurt the beneficial bacteria. Vacuuming the gravel and performing regular water changes are positive. You should now reduce the feeding of the fish to whatever they can eat within just under one minute. Feed only every other day. In the long term, this will reduce the available waste on which these worms feed of. Reducing the food will also entice some fish to look for a food alternative. You get the idea …

      • Ree says:

        Hi. I am desperate to find out what these thin white thread like things are in my freshwater tank. When I put the light on, they appear to rise to the top of the tank. They look like white inch worms. They swim. I have already lost a few fish and now have a loach who is rubbing himself on the rocks at the bottom of the tank. He is bloody red on both sides and his gills have small red splotches. I know that he is in distress. I have dosed the tank with copper green and twice with parasite guard. If you could give me your opinion of the problem, I would be greatly appreciative.

        • Ree says:

          Hi. New development. I replaced the heater with a stronger model. Turning heat up gradually to 80 degrees. The pleco is now attaching itself to the loach and cleaning the wounds on its sides. Is this harmful to either fish?

          • Thilo @ Algone
            Thilo @ Algone says:

            Pleco’s are territorial and can be aggressive. Despite these fish being sold as “algae eaters” pleco’s are actually omnivores. The attaching is not a friendly move, you need to separate the fish.

  16. Cynthia says:

    Hello this is the second time that I notice white clear tiny worms floating and attached to the tank. The first time my fish died and had blood when I took him out the tank. is there a way k can treat the fresh water fish or water? The fish is a beta.

    • Thilo @ Algone
      Thilo @ Algone says:

      The free floating worms are likely planaria and not really harmful. Anchor worms are external parasites, you will spot them being attached at the fish. In both cases, it helps to increase aquarium maintenance and to reduce the amount of food. This certainly helps with planaria as the populations grows with excess organic material. Reducing these factors will reduce the population.

      In case of anchor worms (or other parasitic worms), it would be advisable to use some medication eliminating the parasite. Keep in mind that the worms and the fish death might not be related, but if in doubt, I would treat the aquarium to eliminate all traces.

  17. Claire S says:

    How do I get rid of spaghetti worms in my reef rank? They are keeping the zoas closed with their fine hairs.
    I have many fish. Some one said a 6 line wrasse would eat them but no luck. Please help!!!

    • Thilo @ Algone
      Thilo @ Algone says:

      Terebellids known as Spaghetti worms among aquarists are actually quite beneficial. These worms are detritus eaters and keep things clean in the aquarium. Generally the population is non threatening, but as the detritus increases, so do the numbers of worms. The best way to control them is to improve the environment. The less detritus the lower the worm population. A natural enemy would be the sand sifting star.

  18. Thilo @ Algone
    Thilo @ Algone says:

    Planaria worms seem to be the most likely as they are known to eat worms. These are not harmful to fish, but do indicate a less favorable aquatic environment. Blue Mystery, or Apple Snails, do prefer a nutrient rich environment. You should control the number of snails in the aquarium. As beautiful as they are, too many of them will end up being a pest.

  19. Adam says:

    Hello I have a ten gallon aquarium an my blue mystery snail just laid eggs I noticed a couple days later that one of them had a worm poking into one of the eggs wich are above the water line I removed it and the next day I found more of these little worms with brown/black color heads… can some one please help with info about these worms as far as how to remove them or if they are harmful to my fish aswell or are they only after the snail eggs

  20. Cory says:

    Our fish recently died, it started out with nasty aquarium water & the fish had red spots. It turned into looking as it scum was eating away at the fish. Now that he’s dead I’ve noticed small brown worms at the bottom of the tank. Which kind of worms are these? Could this have been what killed my fish, I tried amoxicillin but how do I make my tank safe again?

    • Thilo @ Algone
      Thilo @ Algone says:

      It is possible, but he worms are very likely not posing any danger to the fish, unless you detected them on the fish. Small brown worms could be planaria and or if you have excess mulm in the gravel, detritus worms. Both are harmless. Deteriorating water causes stress on the fish and increased stress levels reduce the slime coat. Without this protective shield, the fish gets vulnerable. Red spots could be inflammations caused by the less than optimal water conditions. First on the agenda should be to make the water safe for the fish again. This generally solves quite a number of problems.