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.

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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One Fish, Two Fish. Red Fish, Blue Fish

I noticed these worms in my friend’s tank. I cannot figure out what they are no matter what freshwater tank worms I look up. There’s easily a thousand. They hide in a brownish green shell thats stuck to any surface that produces algae as they are eating it. They also seem to let their shell go from the wall, still inside it, at will to move with the current. They seem to eat algae from both ends…. so possibly lacking a full cycle and possibly parasitic? Im somewhat concerned, Ive never seen them before and I, myself, have had tanks… Read more »

One Fish, Two Fish. Red Fish, Blue Fish

I forgot to mention most are under a centimeter the largest I can see is about a centimeter long. The worm itself is white and translucent to transparent.


I am doing a fishless cycle on a new tank and this morning I noticed a very wiggly white worm floating around. I got some used filter media from a LFS about 4 weeks into cycling (at 6 weeks now, almost there!) So I’m guessing that’s where the little bugger came from. Should I just go crazy and treat my tank with chemicals now, before I get fish? Which chemicals do I use? I really dont want to kill 6 weeks worth of waiting, will chemicals harm my BB? I don’t know if I got other hitchhikers, too?


Also, it should be noted that I am using pure ammonia for cycling, there is no fish food or waste in the tank for the worm to feed on, no idea what it’s eating.


I have super small white worms in my fish tank. They’re free floating and I can see them crawling on the glass. Also they’re wicked tiny, they almost look like dust particles floating around. I’ve boiled the gravel, tank, and decorations several times, and they still show up. What is causing this? I’ve never seen/ dealt with what ever it is.


These are detritus worms and do not pose any harm to the fish. Usually they become visible in the water if the oxygen is low in the substrate or the population experiences an explosive growth. Detritus worms are scavengers and feed of decomposing organic waste. Regular maintenance will correct the problem, vacuuming the gravel and adjusting the feeding habits. Most fish see them as snacks, which is not a bad thing overall.


I have little white worms in my tank, that I have never noticed before. They appeared after a gravel vac, and they were free swimming, but quickly disappeared after a few minutes. My fish (platys) are them. I am super nervous that they are harmful to my fish and tank. What should I do?


No worries, it seems they act as a supplement diet to the fish. Likely detritus worms, aka scavengers. These worms thrive on high waste in the substrate, you are doing the right thing to vacuum the gravel. This will decimate them, hope the fish enjoy the snack.


We have small brown worms all over the tank where the water has receded. Are these at all harmful to humans? We are wanting to try and wipe them off the dry part of the glass before cleaning.


These are likely larvae, no problem in wiping them off with paper towels.


There was a white worm swimming around the middle of the tank Water. Like a small round thin worm similar to a short strand of hair maybe half an inch long. Just wiggling through the water. I have kept several tanks over the years and never seen worms. Should I be worried about this and treat the tank or just do a good cleaning? I really want zero worms in there. I am considering emptying tank, replacing gravel and thoroughly rinsing decor. Maybe even adding bleach to the tank and dechlorinating it before adding fish again?


I would assess the situation before taking such drastic action. Worms are more active at night, so turn off the lights, then take a black light to the aquarium and take a look. Generally, a good vacuuming of the gravel will solve minor worm issues. It is also a good starting point and first step.


I noticed a hair thin white worm crawling inside my tank on the glass eating a bottom feeder tablet. Im sure it is due to my tank being not cleaned in a while. The problem is my snails had babies and im scared to clean the tank because i dont want to kill all the baby snails the size of bread crumbs so i was waiting as long as i can for them to get bigger i can barley see them now. Is there a way to clean the tank without killing all the snails and are these worms harmful?… Read more »


Your are correct on the worms, and no they are not harmful. Snails are contributing to the tank problem and less than optimum water quality. Few things to consider; reduce the amount of food, skip a day or two and feed only very little. Perform water changes at 10% every week. Try to add polyester fibers (pillow filler) to your filter, and replace every few days. Once the snails get bigger, you might want to vacuum the gravel as you do water changes.


It would be very helpful to have some pictures of the different types of worms accompanying this article.


Thank you for this suggestion, we will look into it.


I observed some red round worms while washing my tank. What am i supposed to do?
Can they be fatal for my fishes ?


The worms are not harmful, but if they show up in greater numbers it would indicate a water quality issue. These worms feed of decomposing organic matter in the substrate, and these accumulations can cause problems, not the worms, they are just an indicator.


Hi! I noticed a very small and wiggly white worm in my tank today. I saw that a usual suggestion for getting rid of the worms is feeding less, but if I don’t want to do this is cleaning the tank more regularly an option? Should scrubbing everything down be able to get rid of them? Thanks!


The worms can be seen as beneficial as they help to eliminate waste in the substrate. If you are not bothered by a few worms, don’t worry. More cleaning, especially vacuuming the substrate will reduce the worm population. Cleaning the entire tank is overkill, it will cause more stress on the fish than a few worms will. Nevertheless, the overfeeding is not just because of the worms. The worms are just a symptom if you will. Overfeeding has other consequences such as a rise in nitrates, possible pH fluctuation as the water harbors more and more waste resulting from the… Read more »


I have a question! So I have hair worms on my tank. No I’m it but on the glass and on my cover. Are they harmful to humans???


That is generally quite unlikely, but you should always use caution when reaching into an aquarium. Make sure you have no scratches/ open wounds, and wash your arm after you reached into the water.


I would greatly appreciate some advice. I have a 5 gallon fluval spec V with a betta, sand substrate, and plants. I’ve had a worm population for several months now that I can’t seem to get rid of. They are small white wriggly worms, and my betta does hunt and eat them. I thought they were detritus worms, but they make greenish tube structures on the plant leaves and sometimes the glass, which doesn’t seem to be what detritus worms do. I had driftwood in the tank, but removed it because the tubes proliferated on it. I rinsed the substrate… Read more »


Not sure if the plants are dying because of the worms, or the worm population increases because the plants are dying. Guessing the latter. It is very likely to be detritus or planaria. Both are not harmful and can be controled by improving the plant health.


My pink betta has had difficulty swimming and it is on its side and today I noticed a red spot on the side that is upright so I cleaned her tank and I saw one gray-ish worm in the tank that I have NEVER seen before did the worm infect the fish please help I’ve had my fish for 1 year now and I hope she doesn’t die😢😔


It is visible, so lee likely a parasite. The worm could be detritus or planaria, both are not harmful, but can indicate poor water conditions. In most cases, if you improve the water conditions, the symptoms on the Betta might be resolved as well.


I half a 15 gallon column tank planted. I do regular gravel vacs where not carpeted. Lately I’ve noticed and explosion of worms but I can’t seems to find any reference to them anywhere. I had one person at a lfs in the area tell me they are a sign of a healthy tank. The are white worms similar to detritus worms but thinner and longer. But they have this encasing tube that’s brown and other shades of brown. The worm will stick itself out like a proboscus and then pull it brownish encasement forward. I’d love to send a… Read more »


That would likely be nematodes, check on some images to confirm. Generally not harmful, but if ypu need to eradicate, fenbendazole will do the trick.


Help, I have the little white worms, today I found a huge clutch of them on bottom of tank. Where did they come from? No new fish, nothing new, just fresh water when I completely change my tank. Could they have come through our water? I’m freaking cause I don’t know how they got there!


Detritus worms can exist in the aquarium long before they are noticed. They are not harmful, but will grow at a faster pace when there is more debris/ decomposing organics (fish food/ plant leaves). The population can be controlled/ reduced/ by improving the aquarium maintenance, reducing food, and to make sure that the aquarium is not overstocked.


My last batch of blackworms from the lfs had some brown flat worms tagging along. I removed the ones I could find. I’m assuming they’re planaria or maybe small leeches; about 1/4″ long. Would they be harmful to a guppy or shrimp tank? I’ve been feeding them to my festae fry with little hesitation, but I’m concerned about my more delicate species if any remain with the blackworms. Thanks!


Planaria are not really harmful in general, but that does not apply to shrimp, especially smaller ones can fall prey to some planaria. Fenbendazole will help to get rid of the worms, should they become burdensome.

If it is leeches, watch for round bite marks on the fish. You should notify your local fish store where you bought the batch of blackworms, they might actually know what worms hitched a ride with your last batch.


A few months ago I noticed a thin white worm swimming in my betas fish tank. I cleaned the tank and the worm was gone. Now in the gravel of my tank there is a clump of the little worms. I know that worms that are visible to the human eye are usually not harmful, but is there a way that I can effectively erase these worms from my tank? And what type of worm/parasite could these be, and could they be harmful if it gets too out of control?


There are options; reduce feeding, use a shipon to vacuum the gravel, build a worm trap, or if all fails, you can consult with your pet store and use chemicals (last resort)


I saw an almost transparent looking worm squiggly swimming in my aquarium after I did a gravel vacuum and small water change today. I also saw some smaller ones in the bucket before I emptied it after the second go through with the syphon. Should I be concerned?


These worms a detritus worms. They feed on dead organic material. They are very common and populate the gravel where most of the decomposing fish food/ waste can be found. You might not have noticed them earlier, but they have been in the gravel for some time. Once the population grows, they can venture outside the gravel and become visible. They are not harmful. You can control the worm population by correcting your feeding and maintenance schedule, and by removing the waste from the gravel (siphon). The less decomposing matter, the less worms. Again, these worms are not harmful, some… Read more »

Joshua Jett

What do you use to treat them


You can use a worm trap or resort to commercially available anti worm chemicals (we have no favorite)


Hi, I bought the plants for my new fish tank on Saturday and found two small snails on Monday. They’re inoffensive so wasn’t worried, however this morning noticed a load of tiny grey/brown worms on one plant, the rock underneath it, and one worm on the java fern next to it. I suspect the worms and snails came from the same plant. The worms aren’t moving for now, and I still don’t have fish. Would you think they are cause for concern?

I am happy to email pictures later.


The snails and worms likely hitched a ride on one or more of the plants. There is generally no concern if the worms are visible. A few worms and or snails are not detrimental but could be if the population is getting out of hand. You can pick out the snails by hand for example if you don’t want to take chances.


I have a Beta I got in December. I noticed a few weeks in long white stringy poop which I thought indicated a parasite. I treated the water and then he got better. Fast forward a month and it happened again! Treated the water and it seemed better. Three weeks later it shows up again. I follow all proper cleaning instructions, so I wonder where I’m going wrong? The only variable left is the beta food? Any thoughts? I hate to keep treating my tank and fish with antiparasite medicine every few weeks.


Most internal worms have a life cycle with variuos stages, in case of cammallanus it can be up to 4 weeks. The treatment for parasitic worms needs to be longer than the life cycle of the parasite in order to be successful.