Larger Pond, Larger Fish
There is some truth to the saying: The larger the tank, the larger the fish.
Stephen M. Meyer
Q. I have two fantail goldfish living in a horse trough. They were originally in a 10-gallon tank. They almost doubled their size in a month, or two, after being in the trough! I have heard the saying "The larger the tank, the larger the fish," but is it actually true? Some people say it is, others say no. Could you clear this up?
A. The saying should be: "The larger the ratio of the volume of water to the volume of fish, the larger the fish — especially if the tank or pond is outdoors." Two things are at work here.
The original tank was 10 gallons. Your horse trough is probably between 150 and 300 gallons. So the volumetric water-to-fish ratio of the trough is 15 to 30 times larger than that of the aquarium. Above all else, this means that water quality conditions in the trough are far more stable than in the tank. For this reason, moving your two fish from a 10- to a 100-gallon indoor aquarium would have produced similar results.
But it is also true that if you kept them in the 10-gallon tank and ran 10 gallons of fresh water through it every hour (i.e., a complete and continuous water change every hour, every day) you would notice an equally big difference in your fishes' growth and health. So it is not the confining space that is somehow closing in on the goldfish and stunting their growth. It is water quality — or lack of it in the case of a 10-gallon aquarium. This is why two baby goldfish will grow rapidly in a 10-gallon tank, to a point, and then stop.
But water quality is not the only consideration. Keeping the fish outdoors gives them access to a continuous supply of live foods. Goldfish — like most fish — are grazers and are designed to eat small amounts of food continuously. The once or twice a day feeding schedule they are forced to endure in aquariums is not optimum for growth.
The implications of your observations are obvious. For happy, healthy fish, get them outdoors and keep them in as much water as possible — even if they have to put up with Mr. Ed taking a sip every once in a while.