This strategy involves stocking and quarantining a group of animals at the same time with no additions once quarantine has begun. Adding new animals during the quarantine process is strongly discouraged as it exposes both populations to new pathogens. If the addition of new animals cannot be avoided, the quarantine process must be restarted from the beginning for all animals.

The higher the level of isolation, the higher the level of biosecurity. Ideally, quarantined animals should be in a separate building/room with a dedicated water system. This decreases the risk of aerosol disease transmission (e.g. when a particle/droplet in the air carries a pathogen), as well as direct transmission through nets, hands or water quality testing meters. Lower levels of isolations include isolation of a system, or at a minimum, a separate tank/pond.

In all cases, having dedicated equipment (nets, siphon hoses, buckets…) is strongly recommended. Clearly mark which equipment is to be used for a given quarantine system only and pay extra attention to equipment disinfection.

The presence of signage, footbath(s) and hand washing station(s) close to the quarantine system or building is important to serve as a visual reminder and to increase the engagement of staff and visitors in biosecurity measures.

An advantage of keeping new animals together as a group is to be able to monitor and observe them closely and consistently. It is important to monitor their behavior, feeding response and any mortality that occur.

For example, if fish are seen flashing or scratching on the side of the tank, abnormal swimming or position in the water, visible lesions, and loss of appetite are all important signs of disease that may be caused by an infectious organism.

Daily recordings of feeding activity, behavior, lesions, and mortalities allows for the detection of diseases that may be more chronic and of lower level impact – but still important – in contrast to diseases that kill large numbers of animals in a shorter period of time. Good records will also provide data for future reference.

Another tool to prevent the introduction of some diseases is to treat all new animals – if possible - before they enter a facility or farm, or while they are in quarantine. This will be most efficient and effective if a subsample of animals is tested for pathogens upon arrival (see “Sampling/Diagnostics” below) or if a source is known to have a history specific disease organisms in their stocks.

Common preventive treatments which are generally safe and acceptable for most finfish species include freshwater baths (3-5 minutes) for saltwater fish or the addition of salt to freshwater systems to remove many common external parasites from freshwater fish.

Depending on the species and their relevant diseases, targeted sampling of new arrivals can be a very efficient way to set up a treatment plan. This allows a head start for the treatment, before animals show signs of disease, with a much better chance of a successful outcome. A complete necropsy should be performed on sacrificed animals, but if that is not possible, several diagnostic tests can still be performed using skin scrapes, fin and gill clips, and blood samples. Animals showing clinical signs of disease during quarantine should also be evaluated thoroughly to rule out infectious agents.

Monitoring and maintaining good water quality is essential for a healthy environment which reduces stresses and helps optimize the animals’ natural immune defenses. Check temperature, dissolved oxygen, salinity (if applicable), pH, alkalinity, hardness, and nitrogen cycle chemistry (ammonia, nitrites, nitrates) regularly. Continuous water disinfection through the use of ultraviolet sterilization (or ozone) can help inactivate pathogens in the system water.

Complete system disinfection is recommended after moving animals out of quarantine. Pathogens can persist in reservoirs like sumps, pumps or filters, and contaminate the next group of animals.