HITH; Hole in the Head Disease in Fish

Updated 3-5-13

I have had a lot of questions directed either to me or through the forums that I am active in about Hole in the Head Syndrome, often simply identified as HITH.
I will start off with by saying that I do NOT know everything there is to know about this disorder only to say that I have had a lot of experience in treating and ridding fish afflicted with this problem in my maintenance business over the years. I will also note that I NEVER had an outbreak in the last 25 years of my business due to proper care of fish prone to this disorder. I state this because there is a lot of anecdotal information about HITH or even simply half truths.

What is Hole in the Head Disease?

HITH is an affliction that primarily affects Gouramis, Angelfish, Discus, Oscars and other South American cichlids.
A Cichlid has spots on their head which are called sensory pits. These are a series of fluid filled sacs with tiny hairs that sense vibrations and convert them to electrical impulses. Along the lateral lines, sensory pits are responsible for a fish's equilibrium in the water and look like little pin holes, generally arranged in a cluster.
In a HITH outbreak these sensory pits become eroded with ulcerated lesions which expand and coalesce together, forming larger crater-type lesions. They can extend deeper into the muscle and even down to the skull. These open lesions may become secondarily infected with bacteria or fungi and such secondary bacterial infections that may result in death.

A common theory is that HITH is caused by non-absorption of minerals (such as calcium) and vitamins (primarily Thiamine; Vitamin B1) from the intestine due to heavy concentrations of flagellate parasites, like Hexamita.
It is sometimes believed by aquarists that the protozoan parasite Hexamita caused the skin erosions and therefore treatment is based on attempting to eradicate Hexamita by treatment with Metronidazole.
This is where a half truth does come into play; Often there is a misunderstanding as stated above as to the root causes, however there is also a misunderstanding of what Metronidazole is effective against. In this article I will not go into detail about Metronidazole other to say that it IS effective against many bacterium and parasites, especially intestinally which can effect absorption of important minerals and vitamins, WHICH IS WHY Metronidazole has been effective even when the cause was misidentified. The bottom line is Metronidazole is both antibacterial AND antiprotozoal.
Please read more about Metronidazole in this article:
“How Aquarium Medications Work; antibiotics

Back to Hexamita flagellates, these protozoans main effect as to causing HITH appears to me more indirect as this flagellate protozoan absorbs many of the nutrients needed by the fish, in particular Thiamine and calcium, so removal of these parasites with Metronidazole is certainly worth while (assuming their presence), however this again is just half the problem as correcting nutrient problems is as important or possibly more important.

Thiaminase (an enzyme that metabolizes or breaks down thiamine) is also considered a problem that may be a problem in fish afflicted with HITH. Thiaminase is introduced via feeding fish such as Goldfish/Fathead Minnows, or via bacteria. Thaimase breaks down thiamine, and thiamine is an important vitamin necessary for carbohydrate metabolism and normal neural activity. If "feeders" (goldfish sold for the purpose of feeding other fish) are fed as a large portion of your fish’s diet, it likely will develop a thiamine deficiency.
Although many dry prepared foods contain fish flesh that also contains Thiaminase, however the process required to prepare a dry food/pellet likely destroys this enzyme.

There is not conclusive proof that this is a cause of Hole in the Head as Thiamine derivatives and Thiamine dependent enzymes are present in all cells of an animal organisms body, thus a thiamine deficiency would seem to adversely affect all of the organ systems. However, the nervous system and the heart are particularly sensitive to thiamine deficiency, due to their high oxidative metabolism. This adds questions to this theory in my opinion.
However my own observations certainly lead credibility to this theory.
What is also not clear is why this is more of a problem with certain fish and as well, whether the lack of Thiamine is by itself a cause of HITH. Personally I have found that with gut loading of feeders, improved mineralization and Redox that HITH ceases to be a problem even when feeder goldfish are a major part of the diet.

For a related discussion about HLLE, please see this Aquarium Answers Article:
The Lateral Line in Fish as well as Lateral Line Disease or Head and Lateral Line Erosion (HLLE)

Treatment/Prevention

This is also an area where many half truths are present (and quite honestly much is to be learned as well).
Obviously diet plays a role here, as well as water quality such as Nitrate levels, however what is sometimes missed is the role Calcium (in particular calcium cations) plays in proper osmoregulation and the fish ability to absorb other nutrients.
Often poorly maintained aquariums will see high amounts of acids, poor Redox reduction, high DOC (dissolved organic compounds), and low GH and KH (low GH or positive mineral ions). I have also noted in my service calls that the aquarist (usually with South American fish) was told to keep a low GH; unfortunately this is a misunderstanding of GH, Redox, and the need for calcium!

I would recommend reading this article for further information about the importance of proper Calcium levels in ALL aquariums (even Discus!):
“The Importance of Calcium, GH, KH and Electrolytes in Aquariums”

The Redox Balance also plays a more crucial role in prevention and treatment of HITH, as an aquarium that has a poor Redox Balance will likely never allow for recovery from HITH. Redox is directly influenced by a constant supply of mineral cations (found in part in GH), water changes, and mulm buildup. As well, UV Sterilization has a positive affect on Redox, assuming regular changing of the UV Bulbs (every six months)
Please see this article for more about Redox: “Aquarium Redox Potential/Balance”

Here are some steps to take for cure and prevention of HITH:

• Make regular, frequent but small water changes such as 20-30% per week or even twice per week, The use of a gravel vacuum ensures better removal of decomposing wastes that add acids to the water and lower other water quality parameters such as Redox. It is also noteworthy that both Autotrophic and Heterotrophic bacteria also remove important nutrients from the water column in their life processes.

• Lower nitrates, this is important IMO as nitrate levels should be lower than 40 ppm or better, lower than 20 ppm.
Please read this article about the nitrogen cycle for more nitrate information:
"Aquarium (& Pond) Nitogen Cycle"

• Vary diet, avoid the use of shell fish, either avoid goldfish feeders OR feed goldfish feeders sparingly And gut load feeders with foods such as Spirulina 20 or similar high vitamin and mineral foods. Feed high quality pellets as well (or flakes if the fish will eat them).
However since Freshwater fish do NOT drink the water around them liquid vitamins are less effective, but I would still give them a try as well in severe cases especially (adding vitamin supplements to a food slurry then gut loading is the best way to ensure the fish absorbs them).
Please read this article about fish nutrition for more about “gut loading”:
"Fish Nutrition (Proper Aquatic Nutrition). What ingredients are needed for optimum fish nutrition, growth and health"


As to gut loading feeder fish (in particular goldfish and Rosy Reds); many have attributed the feeding of these fish as well as frozen clams or other shell fish to a major cause of HITH do to the high Thiaminase content of these fish. I will concur with this line of thought based on many clients of mine that refused to feed anything else, however when mineral content of the aquarium was improved (such as using Wonder Shells), Redox was improved, AND gut loading of feeders was performed, HITH problems did not occur, even when these clients CONTINUED to feed feeder fish.
Please note I am not advocating that feeder goldfish that are gut loaded be used as food, only the importance of other factors that affect HITH must be considered, as well it is noteworthy that I and many others generally note better growth of carnivores such as larger SA Cichlids (especially Piscivore Cichlids) when gut loaded feeder fish are a major part of their diet.

• Although GH tests do not give a complete or clear picture of positive calcium ions in an aquariums, it is still a start. Increase GH to at least 100 ppm (better is 200 ppm), Wonder Shells or any other method to constantly supply calcium cations along with water changes help here.
Also make sure your carbonate hardness (KH) is at least 50 ppm (over 100 is often necessary for many fish)

• In moderate to severe cases treatment with Metronidazole may also be necessary as well as other antibiotics such as Nitrofurazone (Furan Two)

Finally a clean WELL filtered aquarium is a MUST.