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Using Salt for Goldfish and Koi

Salt will not keep your pond from freezing during the winter.

By Stephen M. Meyer

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Q. I'd like to take this time to commend you on what a great job I think you're doing. I have a 500-gallon pond stocked with about a dozen healthy goldfish and a koi. I know these stocking levels are pushing it, and so I have plans to expand.

Currently, I overwinter the fish in the pond that is 4½ feet deep. I use a cattle de- icer with good success. However, while I was cleaning my saltwater tank I remembered reading about the addition of salt to aquariums containing goldfish. I was pondering whether or not this would be a good idea in my case. I know saltwater freezes at lower temperatures than fresh, and it's easier for fish to maintain osmotic balance. Also, I read it's better for their mucus coating. It all sounds good in theory, but is it practical?

Also, will the plants in the pond (water lilies and marginals) be adversely affected? If this endeavor is feasible, what kind and how much salt (specific gravity) should be used? Would the salt used for African cichlids be safe? Any suggestions will be greatly appreciated. If all goes well, Chicagoland may have its own Great Salt Lake!

A. Well, my basic suggestion would be to enlarge the pond quickly. If the numbers you provided are correct and my math skills hold up (see the previous letter), your pond is only about 5 feet by 4 feet in area.

My experience suggests that this would be suitable over the long term for two 12-inch koi or about six to 10 mature goldfish, depending on the particulars (but definitely including biological filtration). You are well beyond this threshold if your animals are mature. Of course, if your fish are still very small then there is no immediate problem. But with proper care and feeding things will get very crowded in a few years. It is amazing how quickly goldfish and koi grow.

So I guess if you wanted to keep your 12 fish I would proceed to enlarge the area of the pond. Assuming six 12-inch koi and six adult goldfish are your goal over the long term, I would add about 65 square feet of surface area with a depth that varies from 2 to 5 feet. That would add about 1700 gallons to the pond, for a total of 2200 gallons.

How did I estimate these numbers? I begin with a rule that says there should be at least 1 square meter (about 11 square feet) of surface area per 12-inch koi. So that gives me 66 square feet. Of course, this is only an estimate, so in order not to seem overly precise, I just say 65 square feet. Assuming that the pond floor slopes down from 2 to 5 feet, I also assumed an average depth of 3½ feet. That yields 227.5 cubic feet of water, or 1706 gallons (7.5 gallons per cubic foot). The ratio (by weight) of fish to water would be a little over 0.04 percent.

Some readers might note that I have always argued that from the standpoint of estimating fish load, only the first 2 feet of water depth should count. This is true. So by that standard the ratio of fish to water is about 0.08 percent, which is just slightly below my 0.1 percent maximum. Here I am suggesting extra depth to handle overwintering the fish in a cold climate.

Now on to the salt. I assume you are thinking about this as a way to cut down on the de- icer's electricity use while also keeping the pond ice-free longer. Nice theory, but forget it.

Although salt does lower the freezing point of water, that will not help the fish. In fact, it may make things far worse.

Most of the salt addicts in the pond hobby suggest adding between 0.1 percent — 10 parts per thousands (ppt) — and 0.3 percent (30 ppt) salt to a koi or goldfish pond. This is 1 to 3 pounds of salt per 125 gallons. A 30-ppt mix is just about at the safe limit for goldfish and koi over the long term.

As you can see from the figure, if you add enough salt to produce a 10 ppt concentration, you will drop the freezing point to about 31 degrees Fahrenheit. That is not much gain over the freshwater freeze point of 32. More significantly, the salt you added drops the temperature of maximum density to about 36 degrees, which is 3 degrees lower than pure freshwater. So what?

Under normal circumstances the water at the bottom of an iced-over freshwater pond is about 7 degrees warmer than the water at the top. This is because the maximum density of freshwater occurs at about 39 degrees. This water sinks to the bottom and helps keep the fish "warm" as the pond surface freezes over at 32. When you increase the salinity to 10 ppt the water that sinks to the pond floor will have a temperature of 36, not 39 degrees.

It is quite clear that koi and fancy goldfish do not do well at temperatures that drop below 50 degrees. Even the hardiest pond fish have trouble when water temperatures dip below 39. So a pond with salted water becomes even more inhospitable to the fish. It may, in fact, kill them quite readily.

Now, by the time you reach 25 ppt in salinity the freezing point of water has been lowered almost 3 degrees. That may seem good, but at the same time the temperature of maximum density has been lowered to 30, a drop of 9 degrees below pure freshwater!

A few degrees here or there in the temperature at which the pond water freezes is not going to make a big difference in terms of icing over and gas exchange at the surface. But the drastic temperature reduction in the pond floor water that will most likely kill the fish.

Regarding the plants, yes, many pond plants will be negatively affected by salted water. Many will languish and die, others will simply never flourish.

The "salt fad" for pond fish is a bad idea. It produces no measurable benefits to the animals of any kind. For example, despite what you may have heard, long-term salt additions do not reduce pathogenic bacteria or parasite loads in ponds over the long run. And the slight reduction in osmotic differences between the fish and the salted water does not improve the health or well-being of healthy fish.

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