How Do Goldfish Breathe?

Goldfish absorb oxygen from the water ("breathe") through their gills, like many other types of fish. Gills work differently than the lungs of air-breathing animals, but the goal of absorbing oxygen into the blood is the same for both gills and lungs. Goldfish need oxygen to live and they have to remove oxygen molecules (O2) from the water. Although the water molecule (H2O) contains oxygen, the hydrogen atoms prevent the fish from using water as a source of oxygen.

  1. Gill Structure

    • Goldfish gills are cell-thin membranes called lamellae that have a very large surface area to allow water to have maximum contact with the blood vessels. Other parts of the gills include the gill flaps, which can be seen on the outside of the fish, and the support structure of the gills. Gills are very delicate and not supported by bone; the neutral balance of the water allows them to remain open while the fish is swimming. If taken out of the water, the goldfish's gills will collapse and they will suffocate.

    The "Breathing" Process

    • Goldfish take in water through their mouth then force it over their gills. To do so, the fish take in a mouthful of water, close their mouth, then contract muscles in their throat to force the water over the gills. This process allows dissolved oxygen in the water to enter the blood through a process called diffusion. Diffusion occurs when soluble particles in higher concentration move into an area with lower concentration. In the case of goldfish gills, the water contains a higher concentration of oxygen than the blood in the gills, allowing the oxygen to enter the blood from the water. Conversely, carbon dioxide diffuses out of the blood into the water.

    Oxygen in Water

    • Air contains more oxygen than water. Oxygen enters the water through diffusion with the air, where oxygen in the air is dissolved in the water. For goldfish in bowls, this is the primary way oxygen enters the water. Live freshwater plants can also be used to increase the dissolved oxygen in the water through photosynthesis.

    Helping Goldfish Health

    • To help goldfish get the oxygen they need from the water, there are a few things you should do to their captive environment. Colder water can hold more dissolved gasses, including oxygen, than warmer water. Goldfish are generally considered cold water fish, and can tolerate temperatures as low as 65 degrees Fahrenheit, and the colder water will help hold on to absorbed oxygen. As mentioned, live plants help increase the dissolved oxygen in the water and adding live plants to your aquarium will help (but make sure the plants can tolerate the temperature of the water). Increasing the flow of water in the aquarium will help vent carbon dioxide and increase dissolved oxygen. If you have a hang-on-the-back filter, the water flow may be enough for a small aquarium. For additional flow, you can use a powerhead (small submersible water pump) pointed toward the surface of the water. Finally, the gas exchange takes place where the air meets the water, so provide as large an area for this to happen as possible. This means keeping the surface of the water free from debris and allowing air to pass freely over it.