Everything about hexamitids has been clear for a long time — there is nothing to investigate here.
hexamitids
Almost all cichlids (and not only cichlids) always have hexamitids. These are normal internal parasites, part of the intestinal microflora. They live inside the fish permanently and usually cause no problems. Many fish live with them for years, so clearly there is nothing dangerous about it.
Which exact hexamitids they are does not really matter. Hexamita, Spironucleus, Cryptobia — essentially the same thing under different names. They cause the same problems and are treated in the same way, so there is no point in dealing with taxonomy. For aquarists, the result matters, not the Latin genus.
Problems start under stress: water changes, temperature drops, relocation, aggression, spawning, or simply weakened immunity. At this point hexamitids become active and hexamitosis begins. The parasites were living quietly inside the fish before, and then they suddenly get out of control.
Diagnosis is easy and does not require any special tests. If a fish produces white or transparent feces, loses weight, darkens, and refuses food — this is almost always hexamitosis. These signs have been known for a long time and confirmed by practice; there is no need for a microscope. Scrapes and smears only create unnecessary stress and make things worse.
Treatment is also well known. The main drug is metronidazole. It has been used for decades, sometimes combined with increased temperature, salt, or stronger aeration. If it does not work, the dose is increased or the course is repeated. Some people mix metronidazole into food, others add it to water — there is no fundamental difference.
It is impossible to completely eliminate hexamitids. Even after treatment, parasites remain inside the fish and are only temporarily suppressed. Therefore, treatment is about control, not eradication. The key is to notice symptoms early and not let the process progress.
Hexamitids are often said to be introduced with new fish, but this is not necessarily true. They are already present in the fish. Poor conditions simply trigger their activation. That is why prevention is mainly about good water quality and low stress, not quarantine and isolation.
Overall, hexamitosis is not really a disease but an indicator of poor husbandry. Improve conditions, treat with metronidazole — and everything returns to normal.
Intestinal flagellates of the genera Hexamita and Spironucleusare not an obligatory component of fish physiology.
Their absence does not impair digestion, growth, or survival, which clearly distinguishes them from true symbionts
(Dyková & Lom, 1981; Paperna, 1996).
Experience from aquaculture and public aquaria shows that fish populations free of intestinal flagellates:
exhibit more stable growth;
have lower juvenile mortality;
suffer less frequently from chronic wasting
(Cruz-Lacierda & Subasinghe, 1998).
Asymmetry of the host–parasite relationship
The relationship between fish and hexamitids is asymmetric:
fish can exist without the parasite;
the parasite cannot exist without the host.
This is a fundamental principle of parasitology, applicable to most aquarium parasites
(Paperna, 1996; Burreson et al., 2006).
Therefore, the absence of hexamitids represents a normal and stable condition, not an artificial or “sterile” state.
Why the myth of “normal intestinal microflora” emerges
The idea of hexamitids as a normal component of intestinal microflora arises from conceptual confusion:
Microbiota ≠ protozoan parasites
Concepts related to bacterial microbiota are incorrectly transferred to eukaryotic parasites
(Llewellyn et al., 2014).
Subclinical presence
Low parasite abundance may not cause obvious clinical signs, but this does not make it physiologically normal
(Kent et al., 2004).
Lack of diagnostics
In many systems, the presence of hexamitids is never verified microscopically, and conclusions are based solely on nonspecific signs.
White feces is not a diagnosis
White or translucent feces are not specific for hexamitid infection.
Such signs may occur in cases of:
starvation;
bacterial enteritis;
intestinal inflammation of other etiologies;
intoxication;
liver and pancreatic dysfunction
(Kent et al., 2004).
Without microscopic confirmation, the etiological agent cannot be reliably identified.
Why outbreaks actually occur
Hexamitids do not “activate” spontaneously.
Population expansion is linked to specific epizootiological factors:
introduction with new fish;
high stocking density;
chronic stress;
nutritional imbalance;
impairment of intestinal barrier function
(Cruz-Lacierda & Subasinghe, 1998; Tort et al., 2016).
In this framework, stress is a modifying factor, not the source of the parasite.
Key implication
Hexamitids are neither an obligatory nor a desirable component of aquarium systems.
Their presence should be regarded as a risk factor, not as physiological norm.
Fish do not need hexamitids.
Hexamitids need fish.
References
Dyková, I., Lom, J., 1981. Hexamita salmonis and related flagellates of fish. Folia Parasitologica 28, 153–166.
Paperna, I., 1996. Parasites, infections and diseases of fishes in Africa: An update. FAO, Rome.
Cruz-Lacierda, E.R., Subasinghe, R.P., 1998. Occurrence and control of intestinal flagellates in freshwater fish culture. Aquaculture Research 29, 697–703.
Kent, M.L., et al., 2004. Diagnostic procedures for diseases of fish and shellfish. American Fisheries Society, Bethesda.
Llewellyn, M.S., Boutin, S., Hoseinifar, S.H., Derome, N., 2014. Teleost microbiomes: the state of the art. Frontiers in Microbiology 5, 207.
Tort, L., Balasch, J.C., Mackenzie, S., 2016. Fish immune system: a crossroads between innate and adaptive responses. Fish & Shellfish Immunology 53, 1–12.
Burreson, E.M., et al., 2006. Parasites and diseases of marine organisms. Springer.
