A hospital tank allows you to quarantine sick or injured fish, providing them with a safe place to heal while preventing diseases from infecting your entire pond. Quarantine tanks also offer a way to monitor new fish for ailments before introducing them to your pond. While fish hospitals cannot replace a balanced, well-filtered pond, they are essential for keeping your fish healthy.
Creating a Hospital Fish Tank
Fill an aquarium, stock tank, or tote with clean water treated with a de-chlorinator or conditioner that neutralizes the chlorine and chloramines found in tap water. Add a small amount of water from the main fishpond to reduce stress.
Install a diffused aerator and imbue the quarantine fish tank with ample oxygen to protect sick and injured fish from low oxygen distress. Next, add the recommended dose of fish medicine designed to treat the disease or infection you are fighting.
Acclimate quarantined fish to the hospital tank by placing them in clean, clear plastic bags filled with pond water and plenty of air. Tie off and float the bags in the quarantine tank for about 15 minutes. Over the next five minutes, slowly add the conditioned hospital tank water to the bags. Finally, once the fish are accustomed to the new water, open the bags and let them swim into the tank.
After introducing the infected fish to your fish hospital, monitor them closely for the duration of the treatment. Remove fish immediately if they show signs of increased stress, such as flashing or gasping for air at the surface.
How Long to Quarantine Fish
The duration of your fish quarantine depends on the underlying reason for doing so. If you are introducing new fish to the main pond, keep them isolated for about two weeks to ensure they are healthy and disease-free.
However, if you are treating diseased fish in a hospital tank, wait three to five days after symptoms improve to ensure the illness is fully eliminated before returning the fish to the main pond. Alternatively, you may only need to leave the fish in the hospital tank for a short time before slowly reacclimating it to the main pond. Always read and follow the label instructions for the fish medicine you are using.
The Rochester Institute of Technology recommends using this hospital fish tank method to treat one or two fish suffering from a contagious disease. If most of your fish are infected, you may instead want to perform a significant water change and treat the entire pond at once.
Be proactive and prevent common fish illnesses by avoiding overcrowding. Your pond should have no more than two koi or three goldfish per 200 gallons of water. If your fish population exceeds this density, consider building another pond, rehoming fish, or adding extra filtration and aeration to compensate for the increased waste.
Keeping Your Fish Healthy
Whether dealing with disease, poor water quality, or predator injuries, we can help you provide your finned friends with the best care. Contact us online or call 866-POND-HELP to connect with a pond coach who can answer all your koi fish questions.
Read more about keeping fish healthy in the following related guides:
How to Acclimate Fish to Your Pond
Last Updated: July 10, 2025