Q. I have two questions. First, do fish grow faster in a big aquarium? Second, I have four goldfish in a 10-gallon (38-liter) aquarium and they have not noticeably grown in several months. I feed them one to four times per day. They eat flake food, but refuse shrimp pellets. What should I feed them to make them grow faster?
A. Fish may or may not grow faster in a big fish aquarium or backyard pond depending upon the fish load. Too many fish in a given volume of water results in unhealthy water conditions. The fish feel stressed and so they do not eat well. As a result, much of the energy that might go into growth is wasted.
Four fish in a 10-gallon aquarium would probably be healthier and grow more normally if moved to a 50-gallon (189-liter) aquarium. However, if you then add another 16 fish to that 50-gallon aquarium, you are back where you started, or at least the fish are.
The same problem arises when two small goldfish are kept in a 10-gallon aquarium. At first they do very well. They grow full-bodied and show good color. But at some point growth slows or ceases, colors fade and the fish become lethargic. Long before they reached this point the fish had already outgrown their aquarium. In other words, fish load does not only refer to the number of fish per volume of water but also the size (mass) of the fish.
The best way to ensure normal, healthy, robust growth is to keep the fish load low. To me, that means one goldfish per 20 gallons — maximum! Two in 55 gallons is even better.
I always wonder why hobbyists want their fish to grow rapidly. Studies on fish and other animals have shown that accelerated growth is strongly correlated with premature aging and death. In contrast, fish that receive slightly less fish food than they would like — that is, they are always actively searching for fish food — grow slower but live healthier and longer lives.
Goldfish will eat just about anything, but they are actually vegetarians. Shrimp pellets are just not their thing. You would be better off serving parboiled frozen peas with the shells removed. Place some frozen peas in a dish with water and microwave for a minute or two. Remove the shells and serve.
Also, remember that these fish are grazers. Several small meals per day are better than one big feeding.