How Many Goldfish Per Gallon? How To Calculate It?

If you are looking to buy a tank for your Goldfish, give this a read first. If you just bought your Goldfish from the pet store and are wondering how much space you need to keep it, I have got you covered. There are many rules floating about the internet, telling you how much space you need to keep a single Goldfish, but most are without any logical backing.

All fish don’t like overcrowded aquarium fish tanks or ponds. Similar is the case with goldfish. For goldfish there are two rules, for every 1 inch of goldfish you need one gallon and for every 2 inches of goldfish, you need 2 gallons of water. Another rule is for every 2 goldfish you must have 10 gallons of water.

How Many Goldfish Per Gallon?

I will answer all of these questions, and much more. There are more factors involved in the size of your Goldfish tank than you think. I will walk you through them all. But first, why is tank size important?

Why You May Need a Large Tank for Your Goldfish?

Here are the reasons that you may consider the size of your Goldfish tank, before buying one randomly:

  • Growth Inhibiting Hormones: Ever heard how Goldfish grow up to the size of their tank? You may be in control of the size of your Goldfish, without even knowing it. Goldfish produce certain growth-inhibiting hormones, that limit their growth after they have fully matured. If you are keeping your Goldfish in a small tank, or in less water, the growth-inhibiting hormones become very concentrated. Thus, limiting the growth of your Goldfish. However, if you were to keep your Goldfish in more water, the hormones will not be very concentrated, and your Goldfish will continue to grow.
  • Oxygen: Goldfish, like all other living organisms, need oxygen to live. Lesser water means lesser oxygen content for your Goldfish to breathe in. A larger tank or more water is an obvious solution to this. A large enough tank has enough oxygen for your Goldfish to breathe, without the exhaled carbon dioxide becoming more concentrated.
  • Water Quality: A larger tank, having more water, is obviously better than a smaller tank, with less water, in terms of water quality. The waste and unhealthy bacteria are easily more concentrated in smaller volumes of water. A larger tank is easier to maintain, as water changes are less frequent than with smaller tanks. Your Goldfish can survive well even if you change the water a bit later than normal, in large tanks. But with small tanks, you have to make sure that water changes are regular, or your Goldfish may take all the toxins and waste back in, causing it to become sick and die.
Quality Water
  • Aggression: If you are keeping a lot of Goldfish in a small tank, or other fish with your Goldfish in a small tank, you may witness aggressive behavior among the fish. This is because, like humans, fish also need their personal space. Goldfish need a lot of personal space, and with not enough hiding places available, they can harm the other fish if the space around them is compromised. It is better to keep Goldfish in tanks that have enough space for them to swim about without any intervention in their personal space.
  • Early Stages: In the very early stages of your Goldfish, when the fry is maturing into a fully grown fish, you must provide enough space for the fry. As discussed earlier, Goldfish produce growth-inhibiting hormones, but the fry does not need these hormones. If the water around the fry of the Goldfish is concentrated with growth-inhibiting hormones, the fry will not mature properly and will become stunted, eventually causing them to die. So, for the early stages of your Goldfish, you need to provide them with ample water to allow them to mature fully.

How many Gallons of Water do you need for your Goldfish?

identify the number of gallons of water your Goldfish needs to live in. However, you can follow the simple rule that most people around the globe follow:

One Gallon per One Inch of Goldfish To put it more simply, you need about 20 gallons for a single Goldfish, and for each added Goldfish, you must keep adding 10 gallons more.

One Gallon per One Inch of Goldfish

What are the other Factors Contributing to the Tank Size

One gallon per inch of a Goldfish. To me, this information is not enough and can be highly misleading. There are so many other factors contributing to the size of the tank required for your Goldfish. Let me walk you through them:

  • Water Changes: If you think that just keeping your Goldfish in a very large tank will relieve you of all the other duties you have towards your Goldfish, you are awfully wrong. A large tank will allow you to change the water of the tankless often, but you still need to change it. All the waste, toxins, and decaying matter keep building up in your Goldfish tank, and you must regularly remove it, to ensure the proper health of your Goldfish. If you are a busy individual, and you think that you may not be able to change the water of your Goldfish tank so often, you can certainly opt for a larger tank. Those who do not have enough space for a large tank, do not worry. If you have a small Goldfish tank, as much as 2.5 gallons even, and you are doing water changes as much as once or twice a week, your Goldfish will also live just fine.
  • Size of Goldfish: If your Goldfish is 10 inches in length, and you are keeping it in a tank that is barely a foot or 2 feet in length, your Goldfish may not be very happy. Like humans, and any other pet, you must provide enough space for your Goldfish to swim freely. For a Goldfish as large as 10 inches, you must consider buying a tank that is at least 5 times its length. A 20-gallon tank should be enough for a Goldfish that large, provided that you are not keeping other fish with it too. Apart from water changes, your Goldfish needs space to swim about freely, too.
  • Plantation: Live plants are very good at increasing the oxygen levels of your Goldfish tank, during the day. But during the night, the plants produce carbon dioxide which is very harmful to your Goldfish. Live plants also provide shelter for your Goldfish to hide when they are being chased by other Goldfish, or when they are not feeling like socializing. The number of live plants you keep in your Goldfish tank also contributes to the size of your tank. If the plants are providing enough oxygen to your Goldfish, you may not need an extensively large tank for your Goldfish. But make sure that you do not overcrowd your Goldfish tank with plants, as the carbon dioxide at night can be harmful.
  • Feeding: Goldfish enjoy eating a lot, but they only eat as much food as they can within two minutes of you adding food to their tank. Any food that is not eaten within 2 minutes, should be removed. The uneaten food adds up to the decaying matter and decreases the water quality of your Goldfish tank. If you are feeding your Goldfish more food than they can eat, and you are unable to remove the access food, a large tank may help you. The decaying matter will be less harmful in large tanks, due to water being less diluted with the decaying waste toxins.
  • Number of Goldfish: Keeping 2 or more Goldfish in a tank that barely fits a single Goldfish, is obviously not right. If you have a tank that can clearly fit only one Goldfish, and you wish to buy more Goldish, you must also shift to a bigger tank. As discussed earlier, Goldfish need their personal space, and more Goldfish will also produce more waste. This decreases the water quality of the tank more quicker. So, a large tank will help you in keeping the waste diluted.
  • Filtration: There are a number of filters available in the market. Some may not be suitable for your Goldfish tank. When buying a filter, keep the number of fish or Goldfish that you have in the tank in mind. You need to make sure that the filter can bear the burden of filtering the water for as many Goldfish as you have. A large tank will require a filter with a higher power rating so that the tank is filtered properly.
  • Space: If you enjoy keeping a lot of Goldfish, but only have limited space available, you may need to cut short on the number of Goldfish you are keeping. Or, you can work with all the other factors involved, apart from the tank size, to keep as many Goldfish as you like. But there will still be a limit to it. Choosing the right tank size is important if you live in an apartment, or somewhere where space is always under consideration. Your Goldfish need the space and so do you. It is better to keep fewer Goldfish healthy than to keep a lot of them with not enough space. Buying an L-shaped tank can also help you with the issue of space, or you can keep more than one tank to keep more Goldfish.
  • Type of Goldfish: Most fancy Goldfish have large fins and protruding shapes. The large hoods of certain types of Goldfish make it harder for them to swim and breathe. For this reason, the size of a Goldfish tank also depends on the breed of fish. For fish having difficulty breathing due to their specific shapes and features, such as the lion head Goldfish, you need to keep them in a larger tank than you would keep a common Goldfish of the same size.

The size of the Tank Depends on the Type of Goldfish

The different types and species of Goldfish grow up to different lengths and sizes. Since the size of your Goldfish is the major factor in determining the number of gallons of water you need to keep it, you must also have an idea as to how large each type of Goldfish grows. Do note that fancy Goldfish, with large fins and hoods, may need more space even if they are smaller in size than the common Goldfish.

Type of GoldfishMaximum LengthTank Size for Single
Common15 inches25 gallons
Comet12 inches20 gallons
Bubbly Eye8 inches15 gallons
Oranda10 inches20 gallons
Shubunkins14 inches25 gallons
Telescope11 inches20 gallons
Ranchu7 inches15 gallons
Fantail8 inches15 gallons
Watonai20 inches35 gallons
Ryukin10 inches15 gallons
Lionhead8 inches20 gallons

The surface area of the Goldfish Tank

It has been observed that Goldfish enjoy longer tanks rather than deeper ones. If you are providing a sufficient number of gallons of space for your Goldfish, you must also keep in mind the dimensions of the tank. Goldfish enjoy more space to swim about, and generally do not go into deeper waters. This is where the surface area comes into question. Ideally, a single Goldfish of an average length of 8 inches, enjoys about 100 square inches of tank surface area. Make sure that the tank you are choosing is wider than it is deep.

Advantages of Keeping a Bigger Goldfish Tank

A large Goldfish tank will not only keep your Goldfish happy but will also relieve you of some burdens. Here is why:

  • Less Frequent Water Changes: If your Goldfish tank is larger than required for your Goldfish, as per standards, your Goldfish can live with less frequent water changes. A large tank will take a longer time to be dirty enough for you to clean it. It is also not healthy for your Goldfish if you change the water too much, as it kills off the good bacteria in the water, which is beneficial for your Goldfish.
  • Fancy Goldfish: A larger tank will allow you to not only keep more Goldfish but will also allow you to keep the fancy ones. Fancy Goldfish need a lot of space and are much prettier and more attractive than any other fish. A large tank mat is a good enough excuse for you to buy those fancy Goldfish that you have been wanting to buy for so long.

The Bottom Line on How Many Goldfish Per Gallon?

As much as keeping a large tank for your Goldfish is important, I believe that the other factors altogether are just as important. Not having enough space for a 30-gallon tank should not be stopping you from keeping the Goldfish that you have your eyes on. Taking good care in a smaller tank may be better for your Goldfish, than ignoring their well-being in a larger tank. Many Goldfish who have been known to live as much as 30 years, did not live in large tanks. They were stunted in growth but were fully matured and healthy. If your Goldfish is mature and is of a fairly decent size, you may not need a large tank for it after all. All you need is to provide excellent care for your Goldfish, even if your tank is small.

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