The topic of gas embolism (and degassing) is often overlooked as a topic in aquarium care. However, as cold weather approaches for many of us, we wanted to take a moment to talk about water temperature and gas embolism in the home aquarium. This is especially important for hobbyists who rely upon the gadgets which permit direct water exchanges between tap and tank without a storage interval for the water to “age.""
As temperature increases, water can hold less and less by way of dissolved gases; thus cold water has a very great capacity to hold these gases. Tap water carries a heavy load of dissolved gases; much of which will be quickly released in the form of bubbles when the water is modified by being mixed with warmer water.
When cold water is heated in a pressurized system (such as household plumbing or a water heater), the gas cannot escape - until it goes somewhere, like an aquarium! (More gas can also dissolve in water when it is under pressure). Dissolved gases in the water appear as tiny bubbles all over the aquarium, on decorations, and on the fish itself.
Gills of fish are quite permeable; the dissolved gases can enter the fish's bloodstream through the gills. After a short time, the gases will bubble out of solution inside the body of the fish. This is what causes embolism, and the gas bubbles can potentially cripple or kill fish.
If only a small percentage of the water is changed without aging the water first, it's possible that damage may be kept to a minimum level (though not guaranteed); however, a large water change done this way could have negative consequences.
There is another method you can also use to degas the water. While it takes some time for the gas to bubble out of solution when exposed to regular air pressure, it escapes very quickly if the water is vigorously agitated. Often, this will be achieved by using a nozzle on the hose you are filling the aquarium or bucket with, and aiming a strong jet of water against the side of the container. These methods should also provide enough turbulence to drive off the extra dissolved gases. What you need to look for if doing this is for the water to ""boil"" with bubbles and agitation. The bubbles are from air mixing with the water at the surface, but the agitation they produce is what releases the dissolved gases. Seeing tiny bubbles in your aquarium after a water change means you haven't degassed the water enough. Luckily, this is easy to prevent.
*Some information in this article came from The Simple Guide to Freshwater Aquariums, Second Edition, by David E. Boruchowitz, 2009, published by TFH Publications, Inc.