Not all goldfish are created equally, especially fancy goldfish. Many of these varieties have sacrificed internal function for hunched bodies, long, decorative fins and bulging body parts. Most comet and other long-body goldfish varieties will look like this on the inside (minus the pebble):
Those two grey blobs in the middle of the fish’s body are their swim bladder. This organ is responsible for maintaining a fish’s buoyancy, or its ability to swim in the middle of the water column, without sinking or floating. Now, if you manipulate this beginning body shape and modify it over many inbred generations, you get the following shapes:
However, there are a few things fancy goldfish owners can do to limit buoyancy issues.
Keep Your Tank Shallow
This is especially true for very small fish. Deep water exerts more hydrostatic pressure on tiny fish swim bladders, making it more stressful to switch between the bottom and top of the tank. If your fish is stuck on the bottom or top 1/3 of their tank, they may not be able to manipulate their swim bladder fast enough to make appropriate changes. Switching to a shallower tank may help alleviate this issue.
Minimize Water Flow
Some fancy goldfish are very pretty, but poor swimmers. This makes it hard for them to swim in strong currents, burning additional calories and being more prone to severe weight loss. Use an appropriate size filter for your system and position the outflow away from any corners to prevent downdrafts. Some brands have a handy function to decrease the flow to pick a speed that suits slower swimmers, including bettas.
Feed an Appropriate Diet
Remember that goldfish are physostomous fish, with a duct between their swim bladder and esophagus. They require a good-quality goldfish pellet; try to avoid wasteful flakes. If your fish is prone to floating, switch to a sinking diet. And for the reverse, a sinking fish needs a floating diet. Any buoyancy assistance devices are SHORT TERM only to allow the fish to correct the issue themselves. Do not attempt to rig your own fish without consulting a qualified aquatic veterinarian.
Maintain Your Water Quality
Fish get stressed like all other animals. When they are stressed, some of their homeostatic processes, such as buoyancy, can go wonky. Maintaining your water quality is the #1 thing pet fish owners can do to promote healthy fish.