Contrary to what most believe, goldfish need good filtration. Goldfish produce about 10 times the amount of ammonia or waste as fish of a similar size. Traditionally, people did not use filters in ancient china or japan. Instead they simply changed the water everyday. However, most people do not plan to change the water in a large aquarium everyday because of the large water consumption, water bills, or someday they might want to take a vacation. So the majority of people will need to purchase a filter for their goldfish aquarium or pond. Choosing the right filter with the right GPH or LPH is very important as what is appropriate for most fish is not suitable for fancy goldfish. Do not pay attention to the suggested tank size on the filter's box, GPH or LPH can usually be found elsewhere. There are a few different filtration options.
HOB Filters (Hang On the Back)
HOB filters are the most common and typically most affordable option. These filters hang on the outside of your aquarium. It's important to calculate the right GPH(gallons per hour) or LPH(liters per hour) for your aquarium. For hang on back aquariums use the formula "number of gallons multiplied by 10 = GPH" or "number of liters multiplied by 10 = LPH". For example, if you have a 20 Gal tank then you would multiply 20*10 = 200GPH and you would want to buy an HOB filter with 200GPH. Unfortunately, most HOB filters use cheaply made cartridges that will not be suitable for goldfish. Many goldfish keepers prefer to use Aquaclear brand HOB filters.
Canister filters are filters that go under your tank and water is fed into the filter by gravity, then pumped back into the aquarium. Canister filters are a great option for goldfish because they offer plenty space for biological media or good biological filtration. These filters also produce great water clarity. Because they are fed by gravity, they have to be used with an aquarium stand in order to work. These filters can also be a little pricey, but usually you get what you paid for. The GPH or LPH formula is "number of gallons multiplied by 5 = GPH". For example, if you have a 20 gal aquarium then, 20*5 =100GPH.
Sump/Wet dry filters
Sump filters are very permanent, may require you to drill through your tank, and are typically used in larger aquariums. They typically require some extra thinking or research before they can be installed. Water is fed by gravity from the tank into a sump below the tank, the water is passed through a sponge and other biological media, finally the clean water is then pumped back into the main tank. Sumps also house extra equipment like heaters and thermometers so that they do not have to be shown in the tank. Because of their permanent build, expense, risk, and potential complications, they are not for everyone. However, they are great filters for anyone who has fully researched the concept and is willing to permanently alter their tank. The GPH or LPH formula for sump filters is "number of gallons multiplied by 5 to 10 = GPH". For example, if you have a 20 gal aquarium then, 20*5-10 = 100-200GPH.
Sponge filters are only recommended for advanced or experienced goldfish keepers. Sponge filters are powered by an air stone to draw water in through an internal sponge and are not that powerful. These are a good option for goldfish breeders. Sponge filters should only be used in larger ponds or tub setups because the water parameters will be less likely to fluctuate or become toxic. They will need to be used in conjunction with larger, more frequent, water changes. A minimum of 2-3 90% water changes a week will still be necessary to maintain good water quality.
Now that we've covered the different types of filters, we need to talk about filter media. Filter media is anything you decide to place inside the filter to produce cleaner water. There are 2 main types of filter media that you need to have for a successful and healthy goldfish. The first is a quality sponge that can be rinsed. Filter cartridges, and the filters that use them are not a good option for goldfish because of the massive amount of poop that will get trapped in them. Also, filter floss or cartridges are not easy for beneficial bacteria to stick to and the bacteria can be rinsed away every time you clean the cartridge. Other filter pads or sponges can be bought at many pet stores and cut to the appropriate size for your filter with scissors. A quality filter sponge or pad can be rinsed out because the beneficial bacteria will stick to the sponge like super glue.
The second piece of filter media is biological media and is the most important and is ABSOLUTELY NECESSARY for goldfish. Biological media comes in an assortment of shapes and sizes and provides a safe place for beneficial bacteria to grow. These beneficial bacteria are necessary for keeping healthy goldfish because they consume ammonia, nitrite, and produce the less harmful chemical nitrate. Without a proper place for these bacteria to grow the water can quickly become toxic to the fish. Try to pick the type of biological media that best suits your needs. Whatever the filter, it will probably not come with enough biological media, or none at all, and it will need to be purchased separately. Try to use as much biological media as you can possibly fit into your filter.
Carbon is not necessary for goldfish aquariums and can actually be counter productive. This is because the carbon deprives the beneficial bacteria of nutrients and can lower the number of beneficial bacteria in your tank. Eventually this could lead to unstable water parameters, sick, or dead fish. Carbon was originally created by an older generation of fish keepers who did not fully understand the nitrogen cycle. Why it might be acceptable for other species of fish, it should not be used with goldfish.
Don't Use Too Much Filtration
The biggest problem that most beginner fish keepers have is not using enough filtration. However, many people are inclined to believe "Bigger is Better" and this is not a great idea for your fancy goldfish. Fancy goldfish are not great swimmers. Too much flow in the aquarium can be very stressful to fancy goldfish. No amount of filtration will make up for the necessary water changes to keep your fish healthy.