Anchor Worm on Goldfish – Oh No! I have an outbreak in my fish tank!!

One little shortcut that you take in goldfish keeping may result in one big messy disaster in the fish tank. Seriously… I learned this the hard way but I am glad that my goldfish managed to come out alive.

This is what happened…

I have just bought a new goldfish and it’s looking really pretty and active. I brought it home and put it into a quarantine tank. Yeah, the importance of “Isolate First, Socialize Later“, right?  Well, I definitely got that right by quarantining the new fish.

The problem though was that I let my impatience got the better of me.  3 days and the new goldfish was still looking pretty and active, so I thought “What the heck!” and I introduced it to my main goldfish tank. Are you gasping yet? If not, you really should.

What? No!

Everything seemed nice and rosy, and all my goldfish were swimming, eating and pooping well.  About a week later, I noticed that they started rubbing themselves against the rocks and the substrate. Oh, Oh… Then, red spots started to appear on their bodies, followed by these worm-like creatures.

Anchor Worm on Goldfish

Can you see them? Well, let me show you a close-up.

Anchor Worm on Goldfish Closeup

All 3 of my goldfish were having the same thing! Oh No! I have an outbreak in my fish tank!! These worm-like creatures are anchor worms.

Let’s see what they are and how I managed to deal with them.

Anchor Worm on Goldfish

In spite of its name, anchor worm is actually NOT a worm. It is from a family of the copepod crustacean. Yes, you heard me right. Anchor worm is a crustacean!

The worm-like appearance that you see sticking out from the goldfish is its body. The head is anchor-shaped and is burrowed into the body of the goldfish, feeding on it. The bloody red spots that appeared are the goldfish’s defense system reacting to this external parasite.

Anchor Worm
Anchor worm after being pulled out with tweezers.  The top part is the head.

If left untreated, this external parasite will accumulate all over your goldfish’s body, feeding on it alive. It will also attack the gills causing your goldfish to have difficulty in breathing. The wounds will also prone to other bacterial and fungus attacks.

So how did this parasite get into the tank?

Well, if my story wasn’t obvious enough, this parasite was introduced into my tank via the new goldfish.  Silly me for taking the shortcut in shortening the quarantine period. Note to self: Always quarantine for at least 2 weeks!!!

Anchor worm is a parasite that is very common in rivers and ponds. Many goldfish that are sold in fish stores are bred and raised in ponds, so naturally there is the risk of these fish carrying this parasite with them. I should have known better!

Besides that, plants and driftwoods can carry the same risk. So they should be quarantined too.

How I Treated Anchor Worm on Goldfish Successfully

Pull out the parasite

I used a pair of tweezers to pull out the anchor worms that I could see from my goldfish. The important thing to note here is to secure the tweezers on the parasite nearer to the goldfish’s body. This is to ensure that when you pull, the parasite’s head (which is burrowed in the fish) is pulled out.

You can also use your nails instead of tweezers. My nails were too short to do the job.

Anchor Worm on tweezers
Another picture of anchor worm after being pulled out with tweezers


As all of my goldfish in the main tank were infected, I chose to do the treatment in the main tank. My thought was that if the treatment is successful, the parasite would be wiped off from the tank. So I would not need to do a thorough cleaning and disinfecting of the whole tank and filter which would kill my beneficial bacteria as well.

Updated (4-Mar-2018): How wrong I was to think that I could get rid of the parasite once and for all by treating the main tank. Click here on my lesson learned, and what we should actually do.

Anyway, there are a lot of products out there that treat anchor worm. I was recommended by my local fish store to use Fritz Mardel Clout and it worked!

Fritz Mardel Clout

About Fritz Mardel Clout

This product comes in tablet form and it is used to treat and control external parasites such as anchor worms, ick/ich, leeches, lice, flukes and protozoan parasites. Each bottle of Fritz Mardel Clout comes with 100 tablets, capable of treating 1000 gallons of water.

Mardel Clout

The active ingredients in this medication are Metronidazole (an antibiotic) and Trichlorfon (an insecticide).

Important note:  This medication may be harmful to snails and other crustaceans such as shrimps. So if you have these in your tank, you should remove them before treatment.


1 tablet of Fritz Mardel Clout is used to treat 10 gallons of water.  Take out some water from the tank, then dissolve the number of tablets required in it based on the tank’s water volume.

If you have activated carbon in your filter or anywhere in the tank, you need to remove them before pouring your Fritz Mardel Clout solution in. This is because activated carbon has this capability of removing the effect of medication, making them useless.

My experience with Fritz Mardel Clout

Although the packaging mentioned that one treatment should be adequate and you should see the result after a day, I did not get that. I only started seeing the result after 2 treatments.

Please note that I was still pulling out those suckers that I could see during this process. After 2 treatments with Fritz Mardel Clout, the red spots on the goldfish started to disappear and I did not see those anchor worms anymore.  So I am a happy camper with this product.

Clean Up

After the successful treatments, I did a big water change (50-75%) on my main tank to remove any medication residue. You can also add activated carbon to absorb any remaining medication.  Then, I added aquarium salt to boost up the goldfish’s immune system.

Black Oranda


This is a lesson learned – Never take shortcuts in your goldfish keeping especially on the matter of quarantining a new fish properly. Just a little mistake like this can cause a big headache!!!… which in my case, was the anchor worm on goldfish. This outbreak got all 3 of my goldfish sick, but thank goodness I found the right remedy for it – Fritz Mardel Clout.

If you are having issues with external parasites in your fish tank, I would recommend you give Fritz Mardel Clout a try, and let me know how it turned out.

==> Click Here to buy Fritz Mardel Clout <==


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2 Replies to “Anchor Worm on Goldfish – Oh No! I have an outbreak in my fish tank!!

  1. Hi Alex, fantastic article! =) My sister is a fish enthusiast and this will be very beneficial to her. She has always inspired me to start my own. Now after reading your article and browsing your other articles, it has piqued my interest and greatly considers starting one. With this article, you also equipped me with a great understanding of the anchor worms. I’ll share this with my sister as she may not be aware of it. Super thanks =)

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