Goldfish Tank Size? - Fact Based Goldfish Stocking Advice

What tank size do goldfish require? How many gallons per goldfish do you need? These are all questions that you need to consider before buying a goldfish, and they’re also questions that have no black and white answer. 
Many folks like to claim that you need x amount of gallons per goldfish as a rule thumb, or that you need x amount of gallons a base tank size and an additional x amount of gallons per extra fish, but all that is simply not true. There is no rule of thumb because the tank size needed by a goldfish depends on several factors.
Before I go into the specifics, I will say this: Always try to buy the largest tank possible, no matter how many goldfish you plan on keeping. Even if you only want two goldfish, get the biggest possible tank you fit or afford. A bigger tank means doing fewer water changes, doing less filter maintenance, allowing your goldfish more space to swim, and typically results in overall healthier and larger goldfish. 
Fancy Goldfish: 
Any fat or wide-bodied goldfish. Examples of these would include ranchus, orandas, pearl scales, lionheads, fantails, ryukins, and many others. If the goldfish has a fat body, chubby face, or long flowy fins, it’s likely a fancy goldfish. 
Common Goldfish: 
Any goldfish with a skinny body that is typically a faster swimmer and looks thinner from above. In this category, you can also include comet goldfish or shubunkin goldfish. If the goldfish is thin, fast, and strong, you can consider it a common goldfish. 
Now if you’re looking for the absolute minimum tank size to keep a goldfish in, that answer is not as simple either. If the goldfish is a small to medium size FANCY goldfish, the absolute minimum size that would be okay is a 25-gallon tank. If the goldfish is a small to medium-sized COMMON goldfish, the absolute minimum tank size is 35 gallons. However, if the goldfish is medium to large-sized, or even larger, the fish may require a minimum tank size of 55+ gallons. Now if you do decide to keep your smaller-sized goldfish in a 25-gallon tank, be aware that they may not grow as large as you would like. The smaller tank size may stunt their growth over time and they will not grow out to their true potential. 
Ideally, if you are going to put a fancy or common goldfish in a 25 or 35-gallon tank, you would want the fish to already be somewhat stunted in growth. What does this mean? A stunted goldfish may be hard to identify, but in simple terms, if a goldfish looks like it has adult coloration (bright and developed colors), adult characteristics (large wen or deep belly), or is sexually mature (producing sperm or releasing eggs), but is somewhat small like 2-4 inches, then it can be assumed that the fish is stunted in growth. This means that it was either raised in poor conditions, was not fed very much as a baby fish, or was raised with many other goldfish in a smaller volume of water in which the goldfish release hormones that limit the growth of others. No matter the way the goldfish was stunted, it can be safe to assume that their long-term growth potential is forever reduced, at least to a small degree. Though this may sound like a bad thing, and a lot of the time is, it does make an ideal candidate for a smaller, 25-gallon tank. Most fancy goldfish you see at pet stores are actually stunted in growth and may live well in a smaller tank when compared to a higher quality goldfish from a breeder that is not stunted. Keep in mind that stunted goldfish will still grow, just not as much and not as fast, even in good quality water. This is why if you only have the capacity to have a smaller tank, it’s best you buy a goldfish that won’t really outgrow it (a stunted goldfish). 
Once you are able to determine the appropriate tank size at least 1 goldfish to swim around in and live its days (25+ or 35+ gallons, but the bigger the better), then you can theoretically add a lot more goldfish in that same tank without upgrading the tank size. If a tank is big enough for 1 goldfish to swim around comfortably, then it’s absolutely big enough for 2 goldfish to swim around in, in fact, it’s probably big enough for 5-10 to swim around in comfortably. You can technically add all these additional goldfish, however, the more fish you add, the harder it will be to maintain clean water. Once you establish a proper tank size for a fish to swim in, the only other important factor to keep in mind is the water quality. 
The larger the tank size and the less goldfish you have, the easier it is to maintain good water quality. Fish produce waste and a large water volume dilutes that waste, which means less fish and larger water volumes typically make it much easier to maintain clean water. HOWEVER, clean water CAN be maintained in smaller tanks that are heavily stocked with fish IF the filters are very overpowered and water changes are done very frequently. This is why you could theoretically keep 10 goldfish in a 55-gallon tank if you were changing out a large volume of water every single day and had many filters on the tank. 
In short, if you are willing to buy extra filtration, and do a lot more tank maintenance you can “overstock” your tank. As long as your water parameters are in check (no ammonia, no nitrite, low nitrates) then your system and your stocking are viable. Now of course if you way overstock your tank to the point of the goldfish running into each other at every turn, that will stress them to death, but 2 goldfish in a tank vs 10 goldfish in the same tank can easily happen if both tanks maintain proper water parameters through filtration and water changes. But again, I will say, if you have a very large tank, with very little fish, it will be MUCH easier to keep your water parameters in check. 
If you are trying to grow your fish to a large size, you will need either a larger tank or be willing to more water changes forever. Growing a fish out to a larger size may mean upping the temperature and feeding a lot more food, which means the fish will create a lot more waste. Not only will they produce a lot of waste during this grow-out process, but they will also produce more waste as a larger goldfish, so it’s best to have a 40+ gallon tank if you want to grow out a juvenile goldfish to a larger size.
When buying your tank: buy a longer, more shallow, and wide tank instead of a skinner and taller one. Goldfish like horizontal space to swim around in, and they like lower water pressures too. If the height of the tank is over 2 feet, then it’s likely too tall for the fish. The water pressure at the bottom may be too great and the fish will have to do too much vertical swimming instead of the ideal horizontal swimming. 
As I said before, my golden tank rule is to get the biggest tank you possibly can. It will make your life easier, and if you hate doing a ton of water changes like most people do, it will make fish keeping a better overall experience. As for stocking, I always recommend getting at least 2 fish because fish are social and like having friends. However large your tank is, I recommend buying those initial two fish, understanding the care they need and the water changes they require based on your tested water parameters, then if want more fish, understand that getting an additional fish means 50% more water changes. If you get 2 additional fish, you’ll have to do double the amount of water changes. Let’s say you want to get 4 additional fish, you’ll have to maybe install more filtration and do 3x the amount of water changes. If you’re just getting into the hobby, start small, start easy, and get a hang of things with a low stocking, then add more once you understand the work you’ll have to do. 
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