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Prevent Diseases in Pond Fish Before They Occur

Quarantine fish, keep pond fish density low, ensure pond fish have space to hide, and keep the pond water clean.

By Stephen M. Meyer |

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Q. My two 6-inch orandas, which I keep in a 10-gallon aquarium, have developed a dark rot on their tail fins, and the veins look bloody.

Q. I have about 30 medium-size koi in a 900-gallon fish pond. Several of them look bloated and the scales along their sides stand out. They look like fat pinecones.

Q. Would you please write an article on pond medications? Specifically, could you please describe the optimal pond medicine chest — which antibiotics, anti-parasite drugs, instruments, etc., you keep on hand to treat fish diseases?

Q. For the past year or so I have kept a 20-gallon goldfish aquarium with one lionhead, one redcap oranda, one bubble eye and two black telescope eyes. About a month ago I added several fantails to my 30-gallon goldfish aquarium. Since then almost all of my original goldfish have died of some strange disease. Their bellies get very soft, the scales fall off, and they bleed through their skin. Could the new fish have brought in some parasite or bacteria? What can I do now to save the rest of my fish?

Q. I am going to be building a pond this spring and want to raise koi. What fish disease books should I buy?

A. These excerpts from the mail I receive from readers are just a sampling of the letters I receive each week. Pondkeepers — like aquarists in general — have concerns about fish diseases. And rightly so.

Pick up any book on ponds, koi or goldfish and you will find lengthy discussions (and gross photographs) of the multitude of diseases that plague our aquatic friends. There are lots of fish diseases out there, and fish health problems are widespread in ponds (and those little water containers called aquariums). Indeed, many hobbyists report spending more time trying to diagnose and treat fish diseases than they do involved in any other aspect of backyard pond management.

Therefore, I am not surprised that I am routinely bombarded with letters asking for help in diagnosing and eliminating fish parasites and eradicating bacterial infections. Nevertheless I tend to shy away from allocating too much space to such requests because, despite the impression one might get browsing among the incredible variety of fish drugs displayed in any aquarium store, in most instances diagnosing and treating fish diseases is a losing proposition for hobbyists. Infected animals almost always succumb — as often from the treatment as from the disease.

Correspondingly, I have repeatedly ignored requests to describe the optimal pond fish medicine chest because it creates the illusion that such a thing is of any real value in rearing healthy fish. Prevention is a far more successful strategy for dealing with disease. That is why I focus on water quality management and related "environmental" variables.

I strongly emphasize low (what most would consider very low) stocking levels as the foundation of disease prevention. In fact, visitors to my ornamental ponds who are more accustomed to the densely packed fish ponds commonly encountered in hobbyists' yards often ask whether my ponds have just suffered a disease wipeout. They just assume the relative paucity of fish is the aftermath of some great epidemic. I also admonish readers not to move fish around frequently (as in taking them to shows), or continuously add new fish without proper quarantining.

So, imagine my joy when I came across the following in Bill Love's "Herpetological Queries" column in an issue of Reptiles (one of Aquarium Fish International's sister publications). A reader asked whether it might be harmful to mix different species of tree frogs in a single enclosure. Bill replied:

"A more realistic worry might be the introduction of diseases or parasites among the cagemates, to which some may have little resistance. Keep the density of cage inhabitants low, provide numerous hiding places so the occupants are not crowded together...and keep the water source clean."

Proper health management of animals in captive environments — whether aquatic, terrestrial or arboreal — share some common principles. First, quarantining of new arrivals is essential. Yes, it is a pain in the neck, but new animals may carry pathogens against which your animals may not have strong defenses. This is especially true of goldfish and koi, which are bred in very crowded conditions and shipped from all around the world. You shouldn't expect a koi raised in California to have the same disease exposure history as one raised in Taiwan.

Second, keeping animal density low greatly reduces the chances of disease transmission. Pathogens have to work harder to get to an uninfected animal when there are very few in a very large space. Many parasites cannot survive long without a host, and the time between hosts grows very long as the density of animals decreases. What about those fish stocking recommendations in aquarium books? Forget them! They are always much too high.

Third, set up the pond so the animals feel secure in their surroundings. Places to hide, rest and sleep are essential. Diseased fish will often segregate themselves, and this behavior alone can reduce the likelihood of transmission.

Fourth, keep the water clean. This means clean with respect to pathogens. This has nothing to do with the transparency, clarity or algae content of the water. Clean means pathogen-free. The concentration and variety of pathogens in the water is a direct function of the density of fish in the water. Keep the latter low and the former will also be low.

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Reader Comments

Lystra Joseph    International

1/10/2014 9:51:03 AM

Very good article. We have to restrain ourselves from wanting to buy every nice fish we see. Ummmm

Dot    Hawk Junction, ON

5/6/2011 5:03:17 AM

Great article and information

Allen    Calgary, AB

3/15/2010 9:37:25 AM

Good article.

lilly    cd'a, ID

12/4/2008 10:05:38 PM

My last fish died of these simtoms:little fade,not eating, and not going to the bathroom

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