How to Safely Add Fish to Your Pond
As a veterinarian who regularly treats fish, I frequently get calls like this one:
The keeper of an established pond – filled with beautiful koi and/or goldfish that the owner loves – adds new fish, only to have some or all of the collection come down with disease symptoms a few weeks later.
The pond setup was perfect. The new fish looked healthy. How could this happen?
[caption id="attachment_887" align="alignright" width="300" caption="Koi show tank: Shows, sales, and retail stores are places that fish with differing backgrounds are intermingled. Contact like this creates a disease risk for the individual AND for the established pond back at home."]
To answer this question, let’s look at what occurs when a disease first invades a new population of fish. Initially, the infectious organism (also know as a pathogen) multiplies and infects increasing numbers of fish until there is obvious disease. In veterinary terms, the disease becomes “clinical”. The genetic resistance inherent in some fish allows them to survive with minimal or no signs of infection, while other fish get sick and then recover; however, many others may die. Eventually, the surviving fish population becomes resistant to the organism, and the infection becomes “sub-clinical”. The infectious agent is usually still present, but only in low numbers, and the fish population has developed immunity. Individual fish show clinical disease only if their immunity decreases, as with stress. These fish are now carriers of the disease.
[caption id="attachment_889" align="alignright" width="300" caption="Fish louse (red arrow) on the skin surface of a goldfish. This external parasite causes stress and promotes bacterial infection in koi and goldfish."]
Now, let’s suppose some of these carrier fish are offered for sale, purchased by an unsuspecting pond owner, and added to an existing pond. The new fish carry the disease-causing pathogen, but they show no signs. The infection spreads throughout the pond. In this scenario, the new fish may be the only fish in the pond that show no symptoms of disease, while the established population must deal with an outbreak of the new disease.
[caption id="attachment_890" align="alignright" width="300" caption="Microscopic image of a fish louse (Argulus spp.), a parasitic crustacean"]
In the reverse scenario, uninfected fish may be introduced into a pond where the resident fish have developed immunity and are disease carriers. In that case, the newly added fish may show signs of disease, but the established fish appear to be just fine.
The Doomsday scenario is this one:
New fish are bought and immediately added to an established collection. During the retail process these fish have been exposed to a novel pathogen, but are purchased and placed in the owner’s pond before signs of disease occur. Neither the new fish nor the established population have immunity, and all the fish – old and new – come down with clinical disease.
[caption id="attachment_892" align="alignright" width="300" caption="Ich (Ichthyophthirius multifiliis) organisms (tiny white specks in the water) leaving the gills of a dying goldfish. This single-celled parasite causes serious disease in many freshwater fish species."]
If the pathogen is a “mild” one – like flukes or fish lice – you may not detect signs of disease for weeks to months. However, if the pathogen is more serious (for example, Koi Herpesvirus or Ich), then you may see clinical signs and begin losing fish within days to weeks.
Simply put, there is a big difference between “infection” and “disease”. Often, fish available for purchase have been acquired from numerous sources, intermingled, and exposed to many different disease-causing organisms (bacteria, viruses, and parasites). They may be “infected” and not show signs of “disease”, or they may have survived a disease but remain infected, becoming “carriers” of the pathogen.
[caption id="attachment_894" align="alignright" width="300" caption="Microscopic image of Ich parasite obtained from a skin scrape. Ich is easy to kill with salt, but requires an extended duration of treatment due to its elaborate life cycle. Note the parasite's horseshoe-shaped nucleus. "]
The risk of introducing disease can be minimized by taking certain steps. The best way to avoid bringing in disease is to have a closed collection: don’t add new fish. If you must bring in new fish, obtain them from a reputable supplier. Quarantine any new fish in a separate water system for a minimum of four weeks before introducing them to your pond. After quarantine and before placing all fish together, put several new fish and several from the established population together in a separate area, and observe for signs of disease. Pre-treat new fish with a salt bath or dip prior to introduction. Perform skin scrapes, fin and gill biopsies on new fish (and established fish!) to determine what, if any, subclinical infections exist before mixing fish from different sources.
[caption id="attachment_896" align="alignright" width="300" caption="Microscopic image of a fluke parasite obtained from a skin scrape. The fluke feeds using mouth parts at its head (A), and attaches to fish with sharp hooks at its tail (B). "]
Adding fish to your pond carries certain risks. However, by taking appropriate precautions, you can avoid problems and increase the size of your fish collection more safely. If you suspect there may be a problem in your pond or aquarium, seek advice from a qualified veterinarian before attempting treatment on your own. Doing so will make an accurate diagnosis easier for your veterinarian and shorten the period before appropriate treatment begins.