Worms in the Aquarium

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.

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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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