III. Internal Parasites: Prevention and Treatment

Internal parasites are something we usually don’t think much about, but thinking about them can save your fish, increase their growth rate and improve their overall health.

Generally, most fish carry a small load of parasites. However, when the fish is ill or under stress, the parasites can and will reproduce to levels that actually harm their host, causing the fish to lose weight, become more susceptible to illness and eventually stop eating and die.

Where Do Fish Pick Up Internal Parasites?

Many fish in the hobby, especially marine fish and the various puffer species, are wild-caught. In the wild, they are exposed to many parasites, yet seem to survive just fine. Once they are captured, however, things take a turn for the worse. The stress of the capture, being bagged and shipped, probably sorted and shipped again, then unpackaged and dropped into display tanks at stores is more than enough to lower the immune system of even the strongest fish, making them more susceptible to the ravages of parasites. And of course, most fish stores have their tanks on a centralized filtration system, which passes along every germ – bacterial and parasitic – to every fish in their tanks. This means that even if the fish was never exposed to any parasite before their arrival at the store, once they are in the tank and on display, they will have been exposed to quite a few nasties.

When you bring home a newly purchased fish, you should always quarantine it, or you risk exposing your current livestock to all those nasty parasites and diseases the fish picked up at the store. For those new to quarantine, this article provides a useful overview. Treating a few fish in a quarantine tank is much easier, safer and more cost effective than trying to treat your entire display. So, quarantine time is the perfect time to treat for internal parasites, to prevent infecting your entire display.

Spotting the Signs

As is the case with humans, most fish already carry a parasite load without showing any external signs. However, if the parasites start to multiply and harm the fish, these are some of the signs you will see:

  • wasting away – often gradually
  • frantic for food (more than usual of course)
  • red or white worms being purged from the anus
  • loss of appetite

The earlier you catch the signs and begin treatment, the higher the chances of a full recovery will be. If the parasite load progresses to a point that the fish is no longer eating, it becomes more difficult to get the proper medications into the fish’s system to treat for the parasites.

Treatment and Prevention

If you suspect that your fish is carrying internal parasites, or just want to begin a regular deworming schedule, there are a few methods from which you can choose. If the fish is still eating willingly, you can feed a commercially prepared antiparasitic food such as Pepto or Discomed.

It is also possible to make your own “de-worming” recipe and soak the fish’s favorite food in it. This method usually has better results, as deworming treatments are often unpalattable, making it unlikely that picky eaters – such as most puffers – will accept it. The food mixed in with the medications should be something that will hold together well when wet and is a tempting and familiar food to the fish. Pelleted foods and frozen foods thawed in the medication mixture are good suggestions.

If the fish is not eating, you can either choose to treat the tank with a prolonged treatment, or by providing shorter baths for your fish. If all hope appears to be lost, the last resort is tube feeding your puffers.

The following post from our hospital contains excellent suggestions for tube feeding by our resident Puffer Queen…

Make sure you use anesthesia. She has had puffers actually bite the catheter in half & swallow it!. Placement can be difficult in that sometimes it just comes out the gills……………but if you have the puffer in your hand you can feel the catheter in its stomach. If you opt for the tube, make a food slurry & add vitamins as well (especially B12). Bob Fenner recommends baby vitamins. She recommended not giving to much as an anorexic fish may have diminished stomach capacity and if over distended the fish could regurgitate….a volume of 3-6 cc for my 4 to 5 inch Dogface was what she thought was about right.

Caution: if you suspect your fish of carrying a very large parasitic load, it may be a good idea to reduce the levels of medication and do several small treatments. The death of a large amount of internal parasites can be very traumatic for the fish. In situations like these, a longer quarantine period and multiple, lower, dosages are a good precaution.

Here are a few of the medications commonly used to deworm fish:

Metronidazole (a freshwater fish medication) and Levamisole Hydrochloride (a livestock dewormer). In the USA, Metronidazole can be found at most fish retailers, and Seachem conviently packages it in a powder form with an included measuring scoop. Levamisole can be found at almost any farm supply store, catalog or online vendor, often in the swine section. Again, try to find the powder form that is soluble in water. Those outside the USA may need to contact their local veterinarian.

If the fish is eating, take a small container or cup and add about a teaspoon of tank water. Add one measure of the Metronidazole powder using the included measuring scoop, and one measure of the Levamisole powder. If you bought tablet form or did not get the Metronidazole with the measuring scoop, you are aiming for around 100mg of each, so you may have to estimate how much of a tablet to use and crush it prior to adding to the water. Stir the water and medication mix until most of the medication is dissolved, and add your food of choice. Let the food soak for at least a few minutes, — longer is better — and feed the normal amount to your fish.

If the fish is not eating, 1/4 teaspoon of the Levamisole powder per 10 gallons of quarantine tank water is a good beginning dosage to treat a fish who is no longer eating. It is believed that Levamisole remains in the water longer at a ph of neutral or below, so you may want to (cautiously) reduce alkalinity to lower your pH if it is higher to help it work better.

Praziquantel. Praziquantel, which, in the US, goes by the brand name Droncit, is another alternative.

As with the other medications, the medication is best provided orally — but prolonged immersion, and more concentrated baths, are viable – albeit less efective and more stressful — alternatives.

TPF Mentor Kelly, AKA Puffer Queen, has been using Praziquantel to treat puffers for years….and uses the dosages recommended by Noga in Fish Disease Diagnosis and Treatment. This is the book most public aquariua use as a reference. Noga’s recommendations are as follows:

Orally: 23mg/pound of body weight x one treatment. This of course would require the fish to be eating or would have to be tube fed.

If the fish is not eating: Baths — 38 mg/gallon for 3 hours. Prolonged Immersion — 7.6 to 38 mg/gallon for 24 hours

Fenbendazole. Fenbedazole is another medication recommended by Noga. He suggests the following dosages for aquatic aplications:

Orally: 11 mg/lb of body weight for 3 days or 23 mg/lb once a week for 2 weeks or by tube feeding 23 mg/lb.

If the fish is not eating: Prolonged Immersion — 7.6 mg/g once a week for 3 weeks.

Garlic. Conflicting reports abound on the Internet regarding the utility of garlic as a de-wormer for freshwater fish. At this stage, it is safe to say that none have been able to draw scientifically solid conclusions. If you have a sick fish, you would be much better off pursuing a more proven medication, in a separate quarantine tank. This article by Steven Pro provides an overview of Garlic, and what we currently know about it.

Pumpkin Seeds. Another natural de-wormer to receive some attention lately has been pumpkin seeds. Some success has been reported. It may be worth trying as a preventative measure, or if other medications are unavailable in your area. See this thread from our forum for more information. Here is a quote from member Nick:

“I hope the pumpkin seed remedy works, and in interest of making it easier to treat fish with, I researched and found this recipe for removing the active compounds-
pumpkin seed dewormer24
1 – Shell and grind up the pumpkin seeds (or buy them at a grocery store).
2 – Mix 500 g of the seeds with three litres of water.
3 – Simmer (do not boil), while stirring, for 30 minutes.
4 – Let cool 30 minutes.
5 – Filter through a cloth, squeezing to remove as much juice as possible.
6 – Reduce over low heat to 150-200 ml.
7 – Make sure to remove oily scum.

This would make a concentrate you could soak famillar foods in. You could probably reduce the recipe a LOT. ”


Using any medication in your display tank always carries some risk. All of the above medications will be harmful to invertebrates, and will decimate micro-fauna, despite the apparent claims of manufacturers to the contrary. A quarantine tank for treatment is very strongly recommended. Be sure also to keep an eye on nitrogenous wastes and be prepared to take action if the biological filter is affected by the treatement.

The Morning After

The following day, you may notice your fish purging dead or dying “worms” from their anus. They may not eat right away, as the dying parasites will likely be affecting their digestive system. However, they will likely recover quickly and be back to their normal selves within a day or two. Most fishkeepers that have dewormed their fish report a noticable growth spurt in the following weeks and months — even in adult fish that were thought to have been fully grown!

After dosing any fish, either via food, tube feeding or treating the water, do a large water change with a throuough gravel vaccuum the following day. This will help clean up dead and dying parasites the fish will purge, and help prevent reinfestation.

Ongoing Treatment?

The debate of whether fish, or any other animal for that matter, should be dewormed on a regular basis, only when showing signs of parasites or if they should be treated at all is an ongoing one. Some fishkeepers deworm every 4 months or so as a preventative. However, with a sensible initial deworming and quarantine protocol, this should not be necessary. As always, observe your aquatic charges, and let their health be your guide.

Published by

Amy J, AKA Myaj

<p>I first kept goldfish as a child, not many survived, but at least my parents figured out a bowl just wasn't going to cut it and got me a 10 gallon. My goldie "Freddy" grew to a huge size with gorgeous, flowing fins, he was breathtaking. Upon Freddy's untimely death when I was 13, I retired the tank to a corner in the basement. </p> <p>In high school, my boyfriend talked me into visiting a local fish store and out came that 10 gallon tank. I tried every "typical" first time tank mix, fancy guppies and a betta, bumble bee gobies and green spotted puffers (with a little "aquarium salt" of course, ugh!), mollies and platies and goldfish... nothing ever survived except for three tough-as-nails Black Skirt Tetras.</p> <p>Over the years I kept just those three tetras, any other fish I put in the tank would die within hours it seemed like. Basically I would feed the fish and that was it. Ocassionally I'd dump out the tank, clean all the gravel and refill it. </p> <p>I also have horses, and was a member of a horse forum. As time went on, I began to browse some of their sister forums, one of which was a fish site. As I read and learned, I realized why my fish had never survived and how horribly I was treating my three tetras. Begin water changes!</p><p>Pretty soon I had picked up a 29 gallon tank on a dual stand, which of course led to a second 29 gallon tank for the bottom of the stand. Then a 33xl, then a 60 cube, then a 75, then a few more 10's, then a 20 long, then a great deal on 2 20 highs, then... well, you get the idea!</p> <p>From the start I've loved puffers. As the fish forum I visited was completely revamped, many of the good, knowledgeable members moved on and started their own forum, which is where I met Pufferpunk. As the years went on, the draw of puffers was too much to resist. I always found myself back here reading posts, learning about the different species and how to take care of them. When one day I stumbled across South American Puffers at a store, I was thrilled! I immediatly bought the puffers, moved my fish in one of the 29's to the 33xl and came running to The Puffer Forum for support and advice. Its all been uphill from there! </p> <p>From the start of my "educated" fishkeeping days, my tanks have been strictly planted tanks. Even my one attempt at a reef tank has macro algaes, although its not very pretty at the moment. The fish I keep have mainly been rainbowfish, goldfish and puffers, with some shell dwellers, loaches and assorted catfish thrown in. </p> <p>In addition to my fish, I have two horses, Buddy and Mahli, and an adopted black lab named Sophie, who is the sweetest dog in the world and I'm very lucky to have her.</p> <p>I hope to continue to help make The Puffer Forum a great place where everyone, including myself, can learn and share their experiences with these great fish!</p>