Quarantining fish to reduce risk of illness


An aquatic species expert shares 6 elements to consider in creating a plan that will mitigate stressors and reduce risk of transmitting pathogens.

Updated January 21, 2022.

Acquiring new fish and reintroducing aquatic animals to a marine environment presents risks to their health and the wellbeing of other fish. In a virtual session of the Veterinary Meeting and Expo (VMX), hosted by the North American Veterinary Community (NAVC), Catherine Hadfield, MA, VetMB, MRCVS, DACZM, DECZM, said fish are common carriers of pathogens but having a quarantine process in place can reduce the risk of transmission.

“A good quarantine should reduce comorbidity and mortality, and it should improve animal welfare. It will reduce the infectious disease risk,” said Hadfield.

Hadfield is senior veterinarian at the Seattle Aquarium in Seattle, Washington, and co-editor of Clinical Guide to Fish Medicine with Leigh Clayton, DVM, DABVP (Avian Practice and Amphibian and Reptile Practice), eMBA, DVM, DABVP (Avian Practice and Amphibian and Reptile Practice), eMBA. The book offers a comprehensive resource for the veterinary care of fish.

According to Hadfield, fish should be quarantined when introducing a new fish to an existing collection and when animals return after mixing with other animals or environments because they might have been exposed to pathogens. Quarantine following transport also provides stress relief and allows fish to adjust to new environmental conditions. “There are other benefits as well but those are the ones that really focus on the animal care,” she added.

In her VMX session— “Fish quarantine for the general practitioner 101”—Hadfield discussed 6 core features needed for an optimal quarantine: planning, appropriate isolation, ideal environmental conditions, monitoring, diagnostics and treatment, and maintaining accurate records.

“The details within each of the 6 categories [are] going to vary a lot. [They are] going to vary depending on the fish coming in, the facilities available, the experience of the people involved, and a whole bunch of other factors,” said Hadfield.


Determining the goal of the quarantine is key. “This is going to be very different if you are getting in rainbow trout to stock a fishing lake versus, you’re getting in koi that you want to have as long of longevity as possible,” said Hadfield.

Other factors to consider include species suitability, the source of the fish, transport, and the quarantine duration. “How long should quarantine be? If you’re stuck for a general goal, 30 days is usually an often quoted one,” said Hadfield. However, species with higher risks should go longer, she added.

Veterinary care providers should also be prepared to anticipate the animal’s needs and contingencies.

Appropriate isolation

Isolation boils down to security and considering where the pathogens are coming from, Hadfield noted.

“For quarantine, you really want independent, recirculating systems. That means the water is not coming from other fish systems, it’s not going to other fish systems,” she said.

If working with flow-through systems, Hadfield recommended limiting the water to a single pass. “There’s no risk of disease transmission there,” said Hadfield.

Maintaining space around a quarantine system also is important since many common fish parasites and pathogens can be transmitted by aerosol. Additionally, Hadfield suggested using décor such as plastic plants that can be easily disinfected.

The water itself should also be carefully considered. “The safest water, typically, for [reducing] pathogens is probably going to be municipal or city water that is dechlorinated. You might also be dealing with surface water or groundwater that is treated before use,” said Hadfield. “The highest risk water is surface water, like from a lake, sea, stream that isn’t treated before its given to the fish.”

Outside the quarantine system, equipment such as buckets should be clean and hands should be washed to further reduce the risk of pathogens being transmitted.

Ideal environmental conditions

Fish should always be acclimated to their new environments, according to Hadfield. “There is so much complexity in the water that just taking a fish out of one tank and throwing it into the next tank could be an enormous stressor on that fish,” she said.

Acclimating temperature can be done by allowing water in the transport bag to adjust and gradually adding in water from the new habitat.

Environmental conditions should be ideal for the animal’s life stage. “This is a period that they really need exceptionally good care,” said Hadfield.

This habitat should provide shelter and be placed in a quiet area. A quarantine environment should include appropriate aeration, sponge filters, and ultraviolet disinfection. Water quality also is critical, and ammonia spikes should be avoided.

“You want to make sure that before you add these fish, you have biological filters that are ready to handle the ammonia,” she said.


The health of the fish and the quality of their water both need close monitoring.

“You’re going to be staring at these animals a lot. This is a high-risk time for these fish. You want to look at their behavior. That’s their posture, their position, how they’re moving around in the habitat,” said Hadfield.

Other behaviors to watch include fish separating out from a school, and fish staying lower or higher in the water than they should be. External conditions including skin, fins, and eyes should be monitored for redness, dull coloration, and white spots, and attention is needed to assess the gilling effort of the fish.

“It’s very important to monitor how well they’re eating because they’re transitioning, often, to new diets,” said Hadfield.

For water quality, parameters for ammonia and nitrite, pH, salinity, temperature, and freshwater chlorine must be monitored. “The bare minimum of those are very critical parameters,” she said.

Diagnostics and Treatments

“If you can focus on the first 4 categories…you can avoid potentially getting to [the diagnostic and treatments] category. You can prevent the problems from developing,” said Hadfield.

Fish illness can be diagnosed with various testing methods in live and deceased animals. Physical exams, gill or skin biopsies, oral or gill endoscopies, skin scrape, and PCR testing are most useful with live fish, according to Hadfield. A necropsy diagnosis can be made with photographs, squash preps, stained cytology, cultures, histology, and fish pathology.

“If you are doing these diagnostics, do reach out to labs and pathologists who are used to working with fish. Consult with them on the best and most appropriate way to collect the samples so that you can get the most out of them,” she said.

Once a pathogen is identified, a course of treatment can be tailored to combat it. In treating fish pathogens, common medications include praziquantel, formalin, antibiotics, and copper. Changes in salinity can also provide relief.

Overall, Hadfield said an integrated management plan can optimize treatment options and lead to improved outcomes. She suggested that clients should be encouraged to seek out consult from veterinary professionals and to use medications that are prescribed rather than turning to online forums and over-the-counter products for treating fish.

“There is a lot of complexity in fish treatment,” she said.

Accurate Records

Relying on evidence is key to guiding the care of animals. It is difficult to improve fish quarantine and treatment of pathogens if what is being done is not recorded and assessed, Hadfield noted.

At the National Aquarium, where Hadfield previously served as a veterinarian, data was reviewed and found that the success of fish quarantine was much lower than what the staff wanted, according to Hadfield. “We decided to make a very conscious effort to improve that using those 6 core principles,” she added.

Take Away Points

No protocol is a one-size-fits-all solution. Hadfield recommended using risk assessment to refine quarantine plans for the needs of each fish acquisition. She encouraged veterinarians to weigh the probability of a possible hazard—frequent, likely, or remote—with the level of consequence, such as cost, or severity of how an animal might be affected—major, moderate, or minor—when developing a quarantine plan for fish.


Hadfield C. Fish quarantine for the general practitioner 101. Presented at: Veterinary Meeting and Expo; virtual. January 15-19, 2022.

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