Not everything with a capsule is Cryptococcus. Morphology may resemble it, but conclusions require caution.

Hyphae in fish are not always saprolegniosis. It is important to distinguish colonization from invasive processes.

The examined fish showed no visible signs of disease during life. Mortality occurred predominantly at night. Gills were pale, and the fish were slightly emaciated. Necropsy revealed granulomas in the spleen and/or kidneys. Parenchymal organs were enlarged and anemic. The gross appearance was consistent with mycobacteriosis. However, Ziehl–Neelsen staining of impression smears from spleen and kidney did not reveal acid-fast bacteria. The granulomas were necrotic, a feature commonly associated with mycobacterial infections. ...

Intracellular microorganisms in fish: morphology, localization, and diagnostic interpretation.

Toggle Myth / Reality Fin Rot Is a Common Bacterial Disease Fin rot is considered one of the most common diseases of aquarium fish. Almost every aquarist encounters this problem sooner or later. The disease is usually believed to be easy to recognize even without special equipment. The main symptom is the destruction of the fins. At first the edges of the fins become uneven or frayed, then they gradually shorten. Sometimes the edges appear whitish or cloudy, and in more severe cases redness may appear at the base of the fins. ...

Fish do not need hexamitids. Parasites need fish. Why outbreaks are linked to introduction and imbalance rather than ‘activation’.

Toggle Myth / Reality 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. ...

Non-lethal fish diagnostics explained: why scrapes, smears and examinations are safe — and why severely compromised fish may die after handling.

✔️ If fish are dying in the aquarium for unknown reasons: Transfer all fish to another tank. Investigate what is happening. Mass mortalities can vary: ⚠️ Different species are dying simultaneously. Different genera and families have distinct lifestyles, physiology, and immune responses. If multiple species die at once, the cause is more likely water chemistry rather than a pathogen. ⚠️ One species dies quickly and massively. This is also often related to water chemistry — the most sensitive species reacts first. ...

Surgical Procedures on Aquatic Animals under Anesthesia: Gentle Technique and Cosmetic Suturing Precision and a gentle approach are critical in fish and aquatic animal surgery, especially for rare or valuable specimens. Standard “mattress” or rough sutures commonly used in hatcheries are not suitable—they increase the risk of complications, necrosis, and wound dehiscence. Operating Table Setup The main water box with a pump ensures a constant water flow. The working table has a cutout for positioning the fish on its back. The pump tube is inserted into the fish’s mouth, and water flows over the gills. Wet cloth covers the fish to prevent drying. An anesthetic is added to maintain the animal in a stable state throughout the procedure. The dosage of clove oil is 0.35 ml per 10 liters. Such fish are very sensitive to it. ...

Toggle Myth / Reality Myth “Only Fresh Fish Can Be Examined” There’s a common belief that if a fish dies in an aquarium, an autopsy is pointless. It’s often claimed that you must dissect immediately after death, otherwise nothing useful can be learned. The reasoning goes like this: fish decompose very quickly at room temperature — within minutes, tissues change, organs lose structure, and parasites either die or abandon the host. ...

The “perception loop” of medications: enthusiasm → widespread use → side effects → fear and rejection → rational balance. It’s time to move on to the final stage… Toggle Avers / Revers Medications as a Lifeline Methylene Blue Antiseptic and mild detoxifier. Supports tissue recovery and oxygen balance. Useful for diagnostics and treating fry. Methylene blue is one of the safest and most accessible dyes in fishkeeping. It helps in cases of nitrite poisoning, improves tissue oxygenation, and acts as a general antiseptic. Used for treating fungal infections on eggs, for short baths, and diagnostic staining. Easy to prepare, easy to use — even for delicate or juvenile fish. ...

Let’s say your name is Lena. Or Tanya. Or maybe Alex. It doesn’t matter — what matters is that you’re curious, willing to improve your work with aquatic animals, and finally have access to a microscope (congrats!). You even got some stains? Then let’s begin. ...

Toggle Myth / Reality Misconceptions In most cases, the disease is caused by supersaturation of water with nitrogen, less commonly oxygen. This typically happens when water is heated in closed systems where gases cannot freely escape — such as heat exchangers in power plants or boilers in hatcheries. Air bubbles in the water, on glass, or on plants are dangerous — they can block the gills of fish and cause death. External signs of gas embolism appear a few hours after fish are placed in supersaturated water. The higher the gas saturation level, the faster the symptoms develop and the sooner the fish may die. At lower saturation levels, fish may survive longer, and signs like the location and shape of bubbles become more variable. During necropsy of larger fish, foamy blood may emerge from blood vessels. Other signs include: hyperactivity, hypersensitivity to mechanical stimuli, loss of balance, twitching fins, scale erection, ray separation, corneal opacity, lightening or darkening of body coloration, loss of vision, exophthalmia. Causes may include: excessive oxygen production by plants, overly aggressive artificial aeration, or the use of untreated tap water. The disease develops in conditions of oxygen and nitrogen oversaturation. Oxygen entering the bloodstream in excess forms tiny bubbles that block blood vessels. When bubbles enter capillaries, they are often larger than the vessel’s diameter, and the heart is not strong enough to push them through — resulting in blockage. The diagnosis is based on clinical signs, necropsy findings, and water analysis, specifically the level of dissolved oxygen. Scientific Perspective Gas Bubble Disease (GBD) is a pathological condition in which free gas bubbles form inside the blood and tissues of fish. These bubbles can lead to ischemia, gill dysfunction, vascular rupture, and even death. It is common — even in professional literature — to see oversimplified explanations like: ...

Imagine this. A fish arrives at the aquarium with signs of a possible bacterial infection. There’s an ulcer, some mucus, a bit of redness. The lab technician takes a sample and sets up a bacterial culture. The results? In 3–5 days. So everyone waits. But the fish is sick now. On paper, this makes sense: the culture will tell us who’s responsible and what antibiotics might work. But meanwhile, the infection progresses. Treatment may already be in progress — and the sample was taken too late. The culture comes back with nothing. People start talking about viruses. The real cause slips through unnoticed. ...

Toggle Myth / Reality Myth Description: “Crypt”, “marine ich”, or “white spot disease” — Cryptocaryon irritans is a ciliated protozoan parasite, often visible to the naked eye. Tangs, butterflyfish, and boxfish are particularly susceptible and typically among the first to become infected. The free-swimming infectious stage — theront — most often attaches to gills and fins. That’s why these areas should always be examined first. Once mature, the parasite detaches from the host and encysts on the substrate. Around day 3, the tomont undergoes uneven cell division. Between days 6–9, dozens to hundreds of theronts hatch and actively swim away in search of new hosts. ...