People usually land on this topic after a fish starts acting “off” — hanging back, refusing food, breathing hard, or showing odd marks — and the worry is simple: is this infectious, and will it move through the tank?
Viral disease is one possibility, but it’s also one of the hardest to confirm at home. Many “viral-looking” problems overlap with poor water quality, parasites, or bacterial infections. The safest approach is quiet, methodical: stabilise the environment, isolate what you can, and treat the situation as potentially contagious until a qualified diagnosis is made.
What viral fish diseases are (and why they matter)
A fish virus is a microscopic infectious agent that uses the fish’s cells to replicate. In ponds, farms, and sometimes home aquariums, viruses can spread through water, contact with infected fish, shared equipment, and the movement of fish between systems. Some cause subtle illness; others trigger rapid losses in groups.
It helps to separate two different worlds:
- Serious, internationally listed diseases that mainly concern aquaculture, wild fisheries, and border biosecurity (often “notifiable” and managed under official programs).1, 7
- Everyday aquarium problems that may be viral, but are more commonly driven by husbandry stressors and secondary infections.
Common viral diseases you’ll see named (and what they affect)
The names below come up often in searches. They are real diseases, but most are primarily discussed in the context of aquaculture, surveillance, and national biosecurity rather than routine home-aquarium diagnosis.
Infectious salmon anaemia (ISA)
ISA is a disease of Atlantic salmon caused by infectious salmon anaemia virus (ISAV). It’s a major concern for salmon aquaculture and is treated internationally as a high-consequence disease.1, 3
Infectious haematopoietic necrosis (IHN)
IHN is caused by infectious haematopoietic necrosis virus (IHNV), a novirhabdovirus that affects salmonids (such as trout and Pacific salmon species).6
Viral haemorrhagic septicaemia (VHS)
VHS (infection with viral haemorrhagic septicaemia virus, VHSV) affects a range of susceptible fish species and is internationally listed due to its ability to cause significant disease and spread via trade and movements of fish and water.4, 5
Spring viraemia of carp (SVC)
SVC is an internationally listed disease caused by spring viraemia of carp virus (SVCV). It primarily affects carp and other cyprinids (including koi and goldfish), and outbreaks are often associated with cooler-to-moderate water temperatures typical of spring conditions.8, 9
A quick correction: “Senegal Parrot” isn’t a viral fish disease
The phrase “Senegal Parrot” appears in some recycled online content, but it is not recognised as the name of a viral disease in fish. If you saw it in a shop listing or forum post, it may be a misunderstanding of a fish variety name, or a mislabelled disease claim. When in doubt, lean on recognised disease lists and official diagnostic guidance rather than informal naming.1, 2
Signs that can fit viral disease (but also many other problems)
Viruses don’t announce themselves neatly. The same outward signs can be produced by water-quality crashes, parasites, bacterial disease, aggression, or chronic stress.
Still, these patterns are worth treating seriously, especially when they appear in more than one fish:
- sudden lethargy, drifting, or persistent hiding
- loss of appetite across multiple fish
- rapid or laboured breathing without obvious gill parasites
- unusual swimming (loss of balance, spiralling, poor coordination)
- haemorrhaging (red patches), bulging eyes, abdominal swelling
- skin lesions or ulcers (often complicated by secondary bacteria)
What to do first (practical containment for home aquariums)
When the cause is unclear, the first goal is to slow spread and reduce stress. This is the same logic used in larger-scale biosecurity: fewer pathways, fewer surprises.10
- Isolate if possible. Move the affected fish to a separate, cycled hospital tank using dedicated equipment.
- Stop sharing gear between tanks. Nets, siphons, buckets, algae scrapers and hands can move pathogens and organic debris.
- Stabilise water quality. Check ammonia, nitrite, nitrate, temperature, pH and oxygenation. Correct gently, not abruptly.
- Pause new additions. Don’t add new fish, plants, or filter media from other tanks until the situation is resolved.
- Record what you see. Note dates, water test results, species affected, and the order signs appeared. This becomes useful evidence if you seek help.
Diagnosis: what’s realistic, and what requires a lab
Most viral diseases cannot be confirmed by appearance alone. A confident diagnosis generally requires laboratory testing (commonly molecular tests such as PCR, plus pathology/histology in some cases), and the right sample taken at the right time in the disease course.2
If you need a definitive answer, speak with an aquatic veterinarian or a fish health laboratory. They can advise on:
- which fish to sample (often freshly deceased or euthanised, not long-decomposed)
- how to keep samples cold and uncontaminated
- which tests are appropriate for your species and signs
Treatment: what helps, what doesn’t
For many viral fish diseases, there is no direct “cure” available to hobbyists. Antibiotics do not treat viruses, though they may sometimes be prescribed by a veterinarian for secondary bacterial infections.
In a home aquarium, the most reliable supports are environmental:
- excellent, stable water quality
- reduced stocking and reduced stress (lighting, aggression, handling)
- strict separation of systems and equipment
- prompt removal of dead fish (to reduce pathogen load and ammonia spikes)
Where a specific disease is suspected that has regulatory significance (for example, diseases listed by national authorities), advice should come from official or veterinary channels rather than “trial-and-error” treatments.1, 7
Prevention that actually works
Prevention is mostly quiet routine. It looks boring, and that’s the point.
- Quarantine new fish. A separate tank for a few weeks catches many problems before they enter your display system.
- Source fish thoughtfully. Avoid mixing fish from multiple systems on the same day, especially when they’ve been stressed by shipping.
- Keep equipment tank-specific. If you must share, clean and disinfect properly between uses.
- Don’t move fish or water between natural waterways. For pond keepers and anglers, this is a common pathway for spreading serious fish pathogens.9
Why aquaculture talks about viral fish disease so often
In aquaculture, viral outbreaks can spread quickly through dense populations and shared water systems, triggering major losses and trade restrictions. That’s why national programs focus on preparedness, reporting, and biosecurity planning for high-impact diseases.1, 10
When to get help quickly
- multiple fish are affected within 24–72 hours
- you see haemorrhaging, severe swelling, or neurological swimming in more than one fish
- rapid deaths continue despite stable water tests
- the fish are high-value (breeding stock, show koi) or you manage a pond with frequent fish movement
An aquatic vet or fish health lab can help you avoid the common trap: chasing symptoms while the underlying cause keeps moving through the system.
References
- Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) – Australia’s national priority aquatic animal disease list
- DAFF – Aquatic Animal Diseases Significant to Australia: Identification Field Guide (5th edition)
- DAFF – Australia’s National List of Reportable Diseases of Aquatic Animals (endorsed June 2025)
- WOAH – Viral haemorrhagic septicaemia (disease information page)
- WOAH – Aquatic Animal Health Code: Infection with viral haemorrhagic septicaemia virus (Chapter 10.10)
- WOAH – Aquatic Animal Health Code: Infection with infectious haematopoietic necrosis virus (Chapter 10.6)
- WOAH – Infectious haematopoietic necrosis (disease information page)
- WOAH – Spring viraemia of carp (disease information page)
- USDA APHIS – Spring Viremia of Carp Virus (SVC) information page (last modified July 30, 2025)
- DAFF – Aquatic animal health (biosecurity, preparedness and disease information hub)
Veterinary Advisor, Veterinarian London Area, United Kingdom