Right Size Aquarium For Goldfish

Goldfish and koi are incredibly hardy creatures that can hang on under the most appalling conditions and mistreatment — this is not a tribute to the fishkeeper, but rather the fish.

Goldfish need large aquariums with strong filtration. Via Elma/Flickr


 I read an article in a magazine on goldfish in which the author claimed that a minimum of 50 to 60 gallons was required for a goldfish aquarium. Get real! I had fancy goldfish in a 20-gallon aquarium with no problems. Another thing I do not understand is all the emphasis on partial water changes. I change my aquarium water maybe every four months. I believe in letting nature take control by using an undergravel filter, plants and fish in a natural balance. Dirt and debris are natural products. Please don’t encourage unnecessary and time-consuming work.


I have chosen to answer your letter (even though it was not originally addressed to me) because I probably spend more time arguing with people about overcrowded ponds (and aquariums) than anything else. I can appreciate your frustration at reading information that seems to contradict your experience. Hopefully, my answer will help you and other readers to understand why, despite your experience, you would do well to abandon your practice of heavily loading your aquariums and follow the article author’s excellent recommendations.

First, the ability of fish to struggle along and survive in an unhealthy environment does not validate keeping them under poor conditions. As I have said previously in this column and other articles in AFI, there is an important distinction between fish that survive and those that thrive. Goldfish and koi are incredibly hardy creatures that can hang on under the most appalling conditions and mistreatment — this is not a tribute to the fishkeeper, but rather the fish.

You said you had fancy goldfish. What happened to them? Did they live at least eight years? Did they develop to full size (4 to 10 inches, excluding the tail)? Did they develop full body conformity, color, fin and head detail? Did they spawn every year? Did they remain free from bacterial and parasite problems? If you answered no to any of these questions, then I have made my point.

Second, your 20-gallon aquarium is not even remotely close to being a natural system. Choose any natural pond or stream you like and dip a 20-gallon net into the water. How many fish do you think you will pull out? And how many days of dipping that net do you suppose you would need to pull out just one fish? Even the suggested load of several goldfish per 60-gallon aquarium is dozens of times greater than nature would support. The fact that you have to pump water through an undergravel filter should prove to you that the setup is anything but natural.

You are confusing employing a natural process — biological filtration — with a natural setup. They are quite different. Of course, you are correct that dirt and debris are natural products, but not in the concentration with which they accumulate in an aquarium or ornamental backyard pond overloaded with fish.

Third, crowding is not a visual or aesthetic function. It is a biological function. So, what matters is whether the fish load inhibits proper development of the animals, not whether your aquarium or pond looks “right.” As the fish load increases, the concentrations of pollutants — nitrogenous wastes, particulates, dissolved organic substances — in the water also increase. No filter system returns the water to pristine conditions. The best filters can only slow the degradation of water quality. So, the greater the number of fish, the worse things get more quickly.

As the dissolved and particulate organic carbon content of the water rises, the bacterial load in the water increases. Fisheries studies show quite clearly that the bacterial load in rearing waters — including pathogenic bacteria — rises proportionately with increasing fish loads. The exact same findings hold for parasites: the greater the fish load, the greater the parasite density. Because a fish’s immune system can only handle so many challenges in a period of time, the risk to your fish increases every time the fish load increases. Crowded aquariums and ponds are simply disasters waiting to happen. Indeed, the epidemics of bacterial disease that sweep through backyard koi ponds every so often and wipe out many fine animals are a direct result of overcrowding.

Lastly, there is the physiological effect of crowding on the animals themselves. This is an internal (hormonal) response. It can affect feeding and, therefore, growth. This, in turn, can affect physical development, sexual maturation, breeding and so on.

All of this begs the question of what is a crowded aquarium or backyard pond? There are no absolute rules. I have, however, been gathering information on goldfish and koi fish setups that are successful (based on the questions I asked above) and those that have had serious problems. I offer the following rules of thumb. Obviously, these are just guidelines. For aquariums or aquarium ponds without either aeration or biological filtration, fish loads should be kept below 3½ ounces of fish per 260 gallons of water. In systems with aeration and biological filtration, such as your 20-gallon aquarium, 35 ounces of fish per 260 gallons is the maximum.

So, if you had three small goldfish with a total weight of 1 ounce in your 20-gallon aquarium with biological filtration, the fish load would be about 13 ounces per 260 gallons (260 gallons divided by 20 gallons equals 13 times 1 ounce of fish) — well below my recommendation. However, in a year or two, and with the right care, those fish could easily have grown to weigh 1¼ ounces or more each! Then the load would be more than 48 ounces (3¾ ounces times 13) per 260 gallons — well over the limit. But long before that, the fish would stop thriving. Interestingly, the article author’s recommendation works out to be between 17 and 35 ounces of goldfish (assuming mature specimens) per 260 gallons of water.

As you can see, your setup might have been sufficient when the animals were young. But eventually you would need a larger freshwater fish aquarium, so you might as well start the young goldfish off right, with plenty of room. From a fish health standpoint, there can never be too few fish an aquarium.

Lastly, in brief, your water changing practice is dangerous. I can guarantee that for the last three of the four months in each of your water-changing cycles, the pH had dropped to below 5.5 and there was no biological filtration at all. The fish were in continuous stress. Weekly water changes are a necessity for healthy and thriving koi and goldfish.

Article Categories:
Fish · Freshwater Fish