Ichthyophthirius is present in every aquarium.
This parasite is believed to be permanently present in the system and becomes active only when environmental conditions deteriorate. Stress, temperature drops, water changes, or conflicts between fish are thought to activate a parasite that had previously remained in a dormant state.
It is widely accepted that Ichthyophthirius forms highly resistant cysts capable of persisting for long periods in substrate, filters, and decorations. Even after prolonged absence of clinical signs, the infection is assumed not to disappear but merely enter a latent phase.
It is often stated that the disease does not necessarily need to be introduced from outside. Instead, it supposedly arises spontaneously when water quality deteriorates, fish are overfed, or maintenance is inadequate. According to this view, stress alone is sufficient to cause the parasite to appear.
Tap water is sometimes described as containing “free-floating cysts”. As a result, any water change is considered a potential source of infection, regardless of water origin or treatment.
Treatment is believed not to eliminate the parasite completely. Medications are said to suppress symptoms only temporarily, while the parasite remains in the system and inevitably reappears under unfavorable conditions.
In classical literature, the dividing stage of Ichthyophthirius multifiliis is described as an encysted tomont (Dickerson and Clark, 1996; Matthews, 2005). I. multifiliis indeed has a cyst-like stage (encysted tomont), but it belongs to the category of reproductive cysts. Unlike cryptobiotic resting cysts of many protozoa, this envelope is thin, temporary, and designed for palintomic division rather than long-term persistence in the external environment.
Persistence is the ability of a microorganism (virus, bacterium, fungus) or a chemical agent to retain viability or structural integrity outside the host for extended periods.
In classical protozoology, the word cyst is used more broadly than the concept of a “highly resistant resting cyst” typical of amoebae. Historically, encystment means “formation of a cellular envelope — regardless of its thickness, function, or degree of resistance” (Corliss, 2001; Kudo, 1954). Thus, in parasitological tradition:
a cyst ≠ necessarily a resting stage
a cyst ≠ necessarily thick-walled
a cyst = a cell enclosed by a wall
Accordingly, the term is used correctly within its scientific context.
Cyst type
Example organisms
Primary function
Wall structure
Metabolic activity
Environmental resistance
Cryptobiotic (resting)
Amoebae, colpodids
Survival under adverse conditions, dispersal
Thick, multilayered, dense
Sharply reduced (cryptobiosis)
High, long-term persistence
Spore / persistent
Myxosporeans, apicomplexans (spores)
Transmission between hosts
Specialized, robust
Minimal
Very high
Reproductive (palintomic)
Ichthyophthirius multifiliis (tomont), some apostomes
Rapid multiple division
Thin, temporary, fragile
High, active division
Low, short-lived
Correct statement:Ichthyophthirius forms a thin-walled, unstable reproductive cyst.
The myth of “ubiquitous” Ichthyophthirius
Ichthyophthirius
Historical note
For centuries, it was believed that living organisms could arise from non-living matter — for example, larvae from decaying meat or microorganisms from water and mud. Experiments by Francesco Redi (1668) and later by Louis Pasteur (1859) conclusively disproved this idea. A fundamental biological principle was established: life arises only from pre-existing life (omne vivum ex vivo). This principle underlies modern microbiology, parasitology, and epizootiology.
Once the life cycle is understood, it becomes clear that I. multifiliis cannot “wait” in an aquarium for years. The parasite lacks a long-term dormant stage capable of surviving adverse conditions or prolonged absence of a host. Therefore, it cannot persist indefinitely in a system without continuous transmission between fish.
Why the parasite cannot simply remain dormant:
The trophont feeds and grows within host tissues;
The tomont actively divides;
The theront actively seeks a host and rapidly dies without one.
None of these stages are adapted for long-term survival without metabolic activity.
If susceptible fish are absent, the parasite’s life cycle terminates naturally.
Therefore:
The parasite cannot persist in substrate for years without a host;
It cannot “awaken” after prolonged dormancy;
It does not arise spontaneously due to deteriorating conditions.
Stress may exacerbate an existing infection but cannot create a parasite from nothing (see historical note above).
Why the myth appears convincing
The subjective impression that “it was always there” arises from common epizootiological errors:
Subclinical carriage: fish may harbor low parasite loads without visible symptoms. Stress or environmental change allows rapid population growth, creating the illusion of sudden appearance.
Incomplete treatment: premature termination of therapy allows residual parasites to persist and later cause relapse.
Repeated micro-introductions: parasites may enter via new fish, water from other aquaria, plants, equipment, or even hands after contact with infected systems.
Diagnostic errors: different skin and gill conditions may be mistaken for “ich”, making unrelated problems appear as recurring infection.
Tap water and “floating cysts”
Claims that Ichthyophthirius is constantly present in tap water are inconsistent with its biology:
Infective stages require a living host within a limited time window (Farley and Heckmann, 1980; Matthews, 2005);
Active stages poorly tolerate prolonged absence of fish (Post, 1987);
Standard water treatment processes are highly unfavorable for parasite survival (Lahnsteiner and Weismann, 2007).
In practice, reports of “cysts in tap water” most often result from misidentification of free-living ciliates or sample contamination.
Key implication
Ichthyophthirius is not an obligatory component of every aquarium.
If detected, it has been introduced and continues to circulate between fish. Infection control is therefore based not on fighting mythical “dormant forms”, but on:
Quarantine of new fish;
Epizootiological control of the system;
Correct and complete therapy;
Prevention of cross-contamination between aquaria.
Farley, D.G., Heckmann, R., 1980. Attempts to control Ichthyophthirius multifiliis Fouquet by chemotherapy and electrotherapy. Journal of Fish Diseases 3, 203–212.
https://doi.org/10.1111/j.1365-2761.1980.tb00444.x
Lahnsteiner, F., Weismann, T., 2007. Treatment of ichthyophthiriosis in rainbow trout and common carp. Journal of Aquatic Animal Health 19, 186–194.
https://doi.org/10.1577/H07-002.1
Matthews, R.A., 2005. Ichthyophthirius multifiliis and ichthyophthiriosis in freshwater teleosts. Advances in Parasitology 59, 159–241.
https://doi.org/10.1016/S0065-308X(05)59003-1
