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Discussion Starter · #1 ·
i recently noticed a small injury in my oscar's head and i'm not sure whether it's just a scrape or the beginning of something much worse. If it is hole in the head disease then how can i cure it? I havn't noticed anything on his tankmates (RD and GT) but i'm still nervous. Will Melafix work or will i have to use something different? What causes this disease? Any help would be much appreciated and im working on getting a pic of his face up. Thanks.

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This is all you have to know about "Internal Parasites and Metronidazole" (Hole in the head). There are some medicine that contents Metronidazole, so you can buy it in drug-store.

Internal Parasites and Metronidazole

"My fish won't eat…"

"I just got a new fish and it won't eat, what should I do?" Unfortunately, this question is one that has plagued many a beginning aquarist, and even the more seasoned. In the onslaught of answers, it is easy to feel overwhelmed. The fish's fast may be an indicator of any number of stresses, including but not limited to parasitic infection, harsh transport, incompatible tank mates, inappropriate water parameters, gastrointestinal maladies, lack of appropriate food, or even (in rare cases) lack of hunger. While each diagnosis deserves consideration, time constraints have narrowed the scope of this article to one frequently encountered cause, intestinal flagellates, and its cure, Metronidazole.

Symptoms of treatable parasites…

Intestinal flagellates are microscopic organisms that use long "tails" called flagella for locomotion. One of the more common species in aquaria is Hexamita, though there may be others. They multiply by binary fission, an efficient means of mass infestation in the correct environment. Flagellates can be found in both the gastrointestinal and reproductive tracts of their hosts. For their prevalence in the hobby, the average aquarist knows very little about them.

Hexamita, an infamous protozoan*

Fortunately, flagellates are both easily recognized and easily treated. Every new fish should be monitored in quarantine before it is introduced to its permanent home. While a fecal sample examined under the microscope can show the flagellates, usually the aquarist recognizes them by a pair of more observable symptoms. During quarantine, look for a lack of appetite accompanied by white and stringy feces. In my opinion, either of these symptoms can warrant a metronidazole treatment by itself, as long as other causes have been eliminated. In other words, if you can eliminate other causes of fasting such as initial transfer stress, bullying by tank mates, secondary illnesses like ich, poor water parameters, blockages caused by inappropriate diet, etc, and the fish still refuses to eat, metronidazole should probably be the first medicinal treatment of the fish. If the white stringy feces appear, metronidazole is a first priority.

Drug Profile…

Metronidazole was originally developed to combat the parasites that cause amebiasis and giardiasis in humans. To put it kindly, both conditions wreak havoc on the human excretory system. To this day, the drug remains useful for both human and veterinary applications. It possesses both antiprotizoal and antibacterial qualities. To alleviate multiplying parasitic populations due to the stress of capture and transport, most imported reptiles (wild caught) receive a metronidazole/panacure cocktail upon arrival, if not before export. Many keepers of more expensive or delicate fish including discus, apistogramma, or wild caught cichlids consider metronidazole a "must have" for their fish medicine cabinet.

Application and Dosage…

There are two ways of administering metronidazole in the aquarium. By far the most effective means is through ingestion. This can be difficult with wild caught fish used to live food, or with a fish that has completely stopped eating. If your fish is still accepting commercially produced food, however, the logical method of getting the medication where it is needed (the digestive system) is through ingestion. To achieve this, dissolve roughly 50 mg (about ¼ tablet) of metronidazole into a tablespoon of hot water. Then soak commercial flake or pellets in the teaspoon so that the medication is absorbed into the food, and feed this diet exclusively for five days. Some companies even manufacture "medicated flake," which usually contains metronidazole, though I feel more confident with the DIY method.

If the fish is not accepting food, the drug must be administered to the fish's gills via the water column. Dissolve 250mg for every 10 gallons of actual tank water volume (a decorated 55 gallon contains just over 40 gallons of water on average). After each water change (see below), administer another full dose. Metronidazole is oxidized after a period of several hours, requiring the aquarist to dose for the full volume of water (not just the amount changed) with each dose. Dose the tank daily for five days.

Regardless of how the medication is administered, several modifications to normal tank parameters accompany the dose. First, raise the temperature to at least 90 degrees. Because metronidazole is a drug designed for human use, it is engineered to have the greatest impact at human basil body temperature (98.6 degrees). Treating a fish in 60 degree water, for example, will most likely be totally ineffective. Remembering that warm water holds oxygen less efficiently than cool water, the aquarist should increase aeration or current within the tank during treatment (to avoid stress or death from heat-related oxygen deprivation).

In addition to increasing water temperature, the hobbyist should increase the water change frequency. Most flagellates have no or at best a very limited means to survive outside of the host, particularly in low organic substrates. As flagellates transfer through fecal exposure, vacuuming the substrate daily will reduce the risk of reintroduction, as well as increasing your overall fish health through hospital-like standards of cleanliness. Daily 50% water changes are not excessive. Additionally, remember to remove activated carbon from any filters, as it can adsorb the medicine, making it unavailable to the fish.

If the diagnosis of flagellates was correct, the fish should manifest signs of improvement within three days. Continue the treatment (medication and water changes) for the full five days, however, to ensure the parasite is completely eliminated. Appetite should increase, and fecal samples should appear normal. Because they may interfere with digestive processes like vitamin uptake, flagellates are frequently associated with head and lateral line erosion, which should be treated separately.

Potential dangers…

Although metronidazole has been shown to be carcinogenic in some tests on lab animals, the potential for curing intestinal flagellates without the drug is not high. A suitable analogy might be the reality that radiation therapy should be avoided unless necessary, but that for certain cancer patients the benefits of radiation warrant the exposure. Similarly, metronidazole should be used in appropriate aquarium situations without much concern.

Anyone planning on using the drug, however, should reject popular rumors that "it's impossible to overdose metronidazole" or that the drug is completely non-caustic. While your fish is not likely to drop dead over a careless dose of the drug, the documented carcinogenic effects demand this drug's responsible use.

At the time of writing, there is no documented evidence to suggest that metronidazole is harmful either to the biofilter or to planted tanks. While there is the potential for a disruption of some parts of the quasi-ecosystem of a "balanced" tank, the major processes such as nitrification seem unaffected.

Prophylactic Use…

Among the most controversial topics in the aquarium hobby is the prophylactic use of medications. Given the potential dangers discussed earlier, metronidazole certainly should not be a regular addition to any tank. I will confess, however, to treating all new fish with metronidazole in quarantine. Especially among certain fish species (e.g. discus, wild caught African cichlids), the presence of flagellates is so common that in my own experience, they are more likely to have the flagellates than not. With a history in reptiles (where the treatment of wild caught animals during quarantine is always practiced), perhaps I am more inclined to such practices than some of the more conservative aquarists. While I have never experienced adverse affects from this practice (the antithesis is true), gauging the long-term effects on the health of the exposed fish is admittedly impossible in my own home aquarium. In the end, prophylactic use is a matter of personal choice whose benefits and risks must be weighed carefully.


Metronidazole is the aquarist's number one tool in fighting intestinal flagellates. It should be on-hand in any situation where new fish are frequently purchased. Metronidazole is the active ingredient in several commercial aquarium products, including Hex-a-mit, Hex Out, and Paragon II. By prescription, its commercial name is Flagyl. Metronidazole is available in powdered form from Seachem, and crystalline form at http://www.fishyfarmacy.com/. While no drug should be used indiscriminately, a better understanding of flagellates and metronidazole will enhance the any aquarists success in the hobby. Happy fishkeeping!

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Article that I posted here is not mine. I found it on NET 2 years ago. But I have some experience with this kind of disease, because in one of my aquariums I keep Discuses (I don`t know is this right plural)

For freakgasolinefightaccident: your picture I kind a blurry so I can`t see well. If your fish has symptoms wont eat, changing color, behave anaemic, fish excrement is spindle white, that are the first symptoms of Hxamita-Internal Parasites. Hole in the head is fact that you didn`t notice that your fish is sick.

You should treat your fish immediately. When discus fish get Hexamita, and you miss first symptoms, and for consequences it get hole in the head, holes are going to stay like scars. Even after cure.

I wish you luck!!!
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