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There comes a time with most ponds when the owners will want to add fish to supplement the original population or replace pets lost to age or predators. It's a perilous step, notes Mike Gannon, which is why he prepares his clients for the occasion early on with words of caution.


  By Mike Gannon

With fair regularity, people for whom I design and install ponds spend less time considering the watershapes than they do the water’s future inhabitants: They wake up in the morning thinking about koi and other fish and go to bed wondering why it’s taken them so long to call me in to create backyard environments for these fascinating creatures.

In the days or weeks it takes us to schedule the process, arrive on site, dig the pond, line it, set equipment, place rocks, insert plants and fill it with water, some homeowners are just giddy with excitement and can’t wait to get to the point where we’ll add fish to the mix.

In these exuberant moments – and while we’re still a looming presence on site – I do what I can to guide these novice pond owners to sensible procedures for future additions of fish to their ecosystems. Sometimes predators may thin initial populations or, for whatever reason, an owner may simply decide to augment the initial stock I’ll be leaving behind.

The pond will change, I tell them, and they’ll need to stay on righteous paths through what can be perilous transitions.


Often, I start the conversation with the setup line for a classic joke: “A guy walks into a bar.”

It’s a bit of levity that disarms them – and then I change course and say, “No, sorry, wrong story. This is the one: ‘A fish is introduced into a pond. It’s introduced into a pond filled with other fish. Those fish have not had contact with other fish for quite some time. The new fish thrives, but all the original ones die within a week.’ ”

That’s not at all funny, not at all, but it’s a sideways approach I use to introduce the serious fact that any foreign element added into an established environment brings with it the risk of contamination. It’s a “joke” my clients will never forget and helps me make it plain that adding new fish to a pond is an inherently risky thing to do – and that there’s no absolutely sure way around the possibility of dire consequences.

Fish that have not been in contact with fish from outside of their pond for a long time, I tell them, have essentially been living in quarantine. They may be perfectly happy and perfectly healthy, but with the passage of time they’ll often lose their defenses against aggressive foreign agents. This is why introducing new fish to an existing population should be a cautious process.

1Now that I have their full attention, I share what I know about lowering the risk when introducing new fish to a pond. And I make it very clear that this is about reducing challenges rather than eliminating them.

For starters, I say, purchasing koi or goldfish directly from a breeder is a great idea. This reduces the amount of “hands” any new fish will have passed through before arriving in a backyard pond, I explain, emphasizing the point that each level of the distribution chain increases exposure to other fish and spells “opportunity” for stress and disease. And if there are no local breeders, I add, many can be found online and will ship their livestock direct to the pond owner’s door.

Another option I suggest is finding a reputable, specialized fish dealer – that is, a business where hamsters, lizards, parakeets, kittens, puppies and other assorted creatures are not present in the same facility. The risk of working with a fish-only retailer is somewhat elevated when compared to purchasing direct from a breeder, I add, but a reputable fish dealer gains his credibility by maintaining a quality facility and selling healthy fish. These businesses can also be useful sources of ongoing advice.

If proper care has been taken in handing and transporting these fish, they’ve generally been through quarantine periods – a big benefit that usually comes with higher price tags. And sometimes these dealers will certify their fishes’ health at time of sale and even offer some type of guarantee.


I also let my clients know that if they’re ever tempted to step beyond the two possibilities I endorse, I can only advise them to beware – and to remember that paying a premium price for fish is a worthy investment because it generally indicates that the fish under consideration are of superior quality:

Do not, I tell them, shop for fish at supermarket-style pet stores. These high-volume, warehouse-style operations often sell fish as a loss leader, essentially giving them away as they sell you high-margin fish food and a host of fish-care products.

Given the fact that many of these places aren’t looking to make money on the fish themselves, they typically purchase low-quality specimens in very large quantities to keep the expense of their loss-leading at a minimum. Moreover, there’s almost never a fish expert on staff, so there’s no advice to be had and no support of any kind beyond which food or care products to buy.

2Then I sum things up: It’s not the fault of the fish, but those on sale in these establishments tend to be stressed, are generally of low quality and all too often are carrying some sort of parasitic or bacterial nastiness as part of the package. These vendors are very often willing to replace dead fish, but that only increases the original risk by adding questionable new fish from the same bad source.

So while a $12 koi may look like a great buy, I tell new pond owners – especially when they know a local breeder is selling similar-looking fish for $60 – that this possible 80-percent savings is likely to cost many times the $48 difference in the long run, because there’s a high probability that the $12 koi has been exposed to some disease or other that will now run rampant through the established fish population and wipe out hundreds or even thousands of dollars’ worth of familiar fish.

So it’ll cost just $12 to make all of a pond’s fish sick – or even wipe them all out. Isn’t it worth paying the extra $48, I conclude, to give yourself a better shot at keeping a pond healthy?


Just as the guy who walks into the proverbial bar opens up a whole world of amusing possibilities, the same holds true for fish being added to a well-established pond – but with a negative potential many times higher than it is with the guy in the joke who just wants to wet his whistle.

We all hope, of course, that any fish we add will acclimate to its new home rapidly and in perfectly good health, becoming a long-term resident and another beloved family pet. The message I seek to transfer is a simple one of caution, responsibility and reasonable expectations when it comes to acquiring new fish and adding them to an existing, populated pond.

Armed with the knowledge that almost anything can happen, good or bad, I teach my clients that they are stewards of delicate ecosystems and need to work with quality fish suppliers – no breezy, impulsive decisions allowed!


Mike Gannon is owner and lead designer at Full Service Aquatics, a pond installation and service specialist based in Summit, N.J. A certified Aquascape contractor, he may be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

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