Goldfish belong to a large family of fish called Cyprinidae, which includes carp and koi. Goldfish are fairly easy to care for and are often the first fish the budding fish enthusiast acquires. Goldfish are roughly dividable into two types – ordinary goldfish with one tail, and fancy goldfish with two tails. If your fish is swimming to the top of the tank, observe it closely to determine whether the behavior is normal or an indication that something is wrong.
Goldfish might not be as smart as dogs or cats, but they are very capable of learning and remembering when it’s feeding time -- they can even learn to press a lever at a specific time to obtain food. If you feed your goldfish at the same time every day, the fish will soon learn to anticipate the meal at that time and will swim to the top of the tank to wait for food.
If you notice that your goldfish swims to the top of the tank or to the side of the tank when you walk in the room, the fish could be responding to your presence, especially if you are the person who feeds it. This is called associative learning: the fish has learned to associate you with food.
If your fish is swimming at the surface and appears to be gasping for air, it may be suffering from hypoxia, or lack of oxygen. This can occur if your tank doesn’t have enough dissolved oxygen to meet the needs of the fish. Test the oxygen levels; less than 4 parts per million requires that you do a partial water change; you'll want to add an aeration source if possible, too, or commit to more frequent water changes. Also check the temperature. If it’s too warm, that means less oxygen is available to the fish. Goldfish water should be between 65 and 68 degrees Fahrenheit and should never rise above 73. Before adding water to a tank, let it reach room temperature.
Swim bladder is an illness that affects a fish's equilibrium and is common in goldfish, causing the fish to float to the top or sink to the bottom, or to float sideways or upside down. Other signs include difficulty in swimming and swimming sideways or upside down. Constipation is the most common cause of swim bladder. Feeding your goldfish green peas, finely chopped melon or orange regularly can help to prevent constipation. Overfeeding can also cause swim bladder. Overfeeding might be the cause if your fish’s symptoms occur immediately after feeding and go away in a few hours.
Leslie Darling has been a writer since 2003, writing regularly for "Mississippi Magazine" and "South Mississippi Living," specializing in food and wine, animals and pets, and all things Southern. She is a graduate of the University of New Orleans.