The number of goldfish you can put into a 10-gallon tank depends on their size, how long you plan on keeping them in the tank, and whether or not you have any other items in the tank. Some experts recommend using the size of the fish to guide you, while others use the number of fish. Either way, you probably don't want more than two adult fish in a 10-gallon bowl — and forget about a 5-gallon fish tank. Even then, it's probably only a good idea to make this size of tank, bowl, or aquarium a very temporary home. Even one fish in a 10-gallon tank might not be a good idea due to a variety of factors that can stunt the fish's growth.
Is anything else in the tank?
A 10-gallon tank only holds 10 gallons of water if you don't put anything else in the tank. By the time you've added a filter, rocks, decorative plants, or a sunken ship for the fish to explore and swim through, you'll have reduced the amount of water in the tank. So, assume that the more items you have in your tank, the less water there will be in the tank to support the fish, and the fewer fish you can safely put in the tank.
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The size of the fish
Another factor that will determine how many goldfish you can put in your tank is the size of the fish. You can put two to four baby goldfish in a 10-gallon tank, but you should move them to a larger tank when they reach 2.5 inches in size. Don't forget to consider the amount of items you'll also have in this temporary home for your fish, which reduces the amount of water in the tank.
You should not keep goldfish in bowls at all. Even a 10-gallon bowl is too small and stunts fish growth. The goldfish bowl as an acceptable habitat for these creatures is a myth and may even make goldfish weak and sad. Imagine being a single goldfish living for years in an empty bowl with nothing to do.
Other experts say a 10-gallon tank is too small for even one adult goldfish, which can live an average of 25 to 30 years. Put in a tank this size, the fish might only grow to half the normal size. Some experts recommend no more than two baby fish in a 10-gallon tank and suggest 20 gallons of water per adult fish when choosing an aquarium size.
Fish tank water quality
The amount of any type of fish you can keep in a fish tank also depends largely on the quality of the water in the tank. If you're keeping a bowl in a child's room, for instance, how often will the water be changed and the bowl cleaned? If the water becomes cloudy, change the water and clean the tank as soon as possible. Goldfish are messy and produce lots of waste. Poor water quality can lead to bacterial infections and fish with a shorter life span.
In addition, they produce and secrete a growth-inhibiting hormone into the water . In a large tank with lots of water, this hormone dissipates and doesn't affect the fish very much. In a small tank or bowl, however, the hormone won't be diluted as much and will reduce the fish's growth.
You should also monitor the temperature of the water, which should be around 65 to 72 degrees. There are slightly different water temperature recommendations for different types of goldfish, such as 68 to 74 degrees for fancy goldfish and 60 to 70 degrees for comets and shubunkins. Make sure you talk to a fish expert about mixing different types of fish in the same tank.
Goldfish tank size recommendations
Consider a larger tank for your goldfish, which holds plenty of water that provides more oxygen for the fish. Because fish are social creatures, think about having at least three in your tank so they can swim and play together. This means a minimum of a 30- to 40-gallon tank. The size will depend on how many other objects you plan on having in the tank and what type of water evaporation rate you expect.
Make sure you know what type of goldfish you are buying and what size they are expected to grow to at maturity. Some goldfish can grow to 1 foot in length. This will help you decide what size tank to buy if you want to start them and keep them in the same tank for the duration of their life.
When it's time to drain and clean their tank, you'll need to be able to keep them somewhere safe during that time, so make sure to have a safe temporary tank or bowl. This can be smaller than your regular aquarium since the fish might only need to be in it for a couple of hours.