Tank Temperature For Goldfish

“Goldfish are cold-water fish, they don’t need a heater”... you’ve probably heard that a couple of times and it’s not entirely true, for a couple of reasons. For one, there are so many different breeds of goldfish and they all have different needs and a different set of ideal conditions. The temperature of your tank should also depend on the current health situation with your fish as well as your goals as a goldfish keeper. Because goldfish are cold-blooded animals, controlling the temperature basically controls their metabolism rate, and there are many reasons why you may want to control that.
*At the bottom of this article, I tell you the exact temperature I try to keep my goldfish at and why.
The first and most important factor to consider when looking for an ideal tank temperature is the breed of goldfish you are keeping. If you have a skinny-bodied goldfish (commons, comets, shubunkins), then you may not even need a heater. Skinny-bodied goldfish tend to be hardier than fancy and fatter varieties and can typically withstand a wider and harsher temperature range. In fact, skinny-bodied goldfish are often left in ponds over winter where the water gets to freezing temperatures (32F, 0C) and still survive without any issue and continue to live long wonderful lives. But if you are looking for an ideal long-term temperature range, a range of 65F-78F (18.3C-25.5C) would be just fine for a skinny-bodied goldfish. The most important thing is to keep temperature changes very gradual. Changing the temperature in your tank only a few degrees a day is recommended. 
If you have a fancy or fat-bodied goldfish, then it's in your best interest to have a water heater. Though some fat-bodied goldfish are able to survive near-freezing temperatures over winter, it is really not ideal for them and can cause long-term health problems. For this reason, I typically advise against putting fancy goldfish in outdoor ponds unless you are heating the pond to an appropriate temperature year-round. Fancy goldfish are more susceptible to swim bladder disorder, gas, bloat, and constipation, and increasing the temperature is the best way to prevent those problems. A higher temperature boosts metabolism allows the goldfish to digest its food easier and faster and that has many positive long-term health benefits like preventing the problems stated. Fancy goldfish can also be more susceptible to some diseases and increasing the temperature can also boost their immune system. Generally, the more highly developed the breed of goldfish is, the greater its need for warmer water temperatures is. For example, a skinner oranda or fantail goldfish might be able to endure some colder temperatures, while a very fat-bodied pearl scale goldfish would not and may quickly develop health problems in colder temperatures. Overall the ideal temperature range to keep a fancy goldfish is roughly 72F-78F (22.2C-25.5C). In this range, they should be able to live a good life with minimal health problems given all other conditions are optimal.  
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Linked here is the water heater I use and recommend.
If you are trying to grow your goldfish to a large size, you will definitely want to warm the temperature. Increasing the water temperature to 75F-78F (23.9C-25.5C) promotes faster and more efficient growth. A warmer water temperature increases the metabolism of the fish, which is the biological force responsible for cell division and growth. Not only does it directly increase the metabolism, but this also allows the fish the ability to handle eating more food without risking their health. The warmer the water, the faster the fish can digest food, meaning they can consume even more, which allows you to do more feedings and push faster growth.
However, two very important things to consider when raising the water temperature higher is that waste production will increase and the oxygen levels will decrease. A higher metabolism means higher production of waste and ammonia, if your filtration system is not powerful enough, this could result in an ammonia spike. If you are increasing the temperature of your tank and feeding more to push faster growth, it is absolutely crucial that you have an overpowered filter system AND that you do more water changes.
Having an overpowered filter system will also take care of the problem of lower oxygen levels. As water temperature increases, its ability to hold dissolved oxygen decreases. In short, this means you need more water agitation and flow to increase the amount of oxygen diffusing into the water. If you raise the temperature in your tank to promote faster growth but do not address the possible issues of lack of oxygen and higher waste production, it will have the opposite effect. Growth may slow and your goldfish’s life could be at risk. Keeping proper levels of oxygen and healthy water parameters is WAY more important for driving growth, but if you can keep those under control, then you can easily increase the water temperature to allow for even faster growth. 
In general, a warmer water temperature will boost the metabolism of a goldfish, and this will also boost the immune system of a goldfish. There are many reasons you may want to boost the immune system of a goldfish, and one may be that you’re quarantining a new fish. Though there are many different opinions on what temperature goldfish in quarantine should be kept at, I have found it’s best to quarantine a new fish at around 80F (26.7C). At this temperature, the goldfish is not too overly stressed from the temperature, and its metabolism is much faster and its immune system is too. Though it is true that at higher temperatures like this, some pathogens might actually grow and have a faster life cycle too, which is actually a good thing if you are quarantining a goldfish and treating it for possible disease. If you are keeping the oxygen levels high and the water parameters in check, the boosted immune system combined with a faster life cycle for potential parasites may help clear the fish of possible disease even faster if you are treating it.
Some keepers recommend quarantining at a temperature of 84F-86F (28.9C-30C) because at this temperature, ick cannot have an outbreak. However, in my opinion, I would not raise the tank to this temperature unless you suspect ick or detect early signs of it. At a temperature that high the goldfish will experience significantly more stress than at a lower temperature, and this could possibly be counterproductive. However, it will really help rid the fish of possible ick so it’s your choice in the end. 
There are many different recommended temperatures to keep sick goldfish at for different diseases. If you want to just boost the immune system of the fish just in case of something, I would recommend raising the water temperature to 78F-80F (25.5C-26.7C). However, if your fish is legitimately sick, you may want to raise the temperature even higher and I would recommend researching the ideal temperature for the specific disease you are battling.
Something you may want to know is that if your goldfish don’t experience winter, and live their lives in elevated temperatures, they will live for a shorter period of time. Because a Goldfish’s metabolism nearly stops during the winter and is slower at lower temperatures, a higher metabolism due to higher temperatures in a way puts the goldfish’s life cycle in fast forward. They will grow faster, they will become sexually mature faster, and they will indeed die faster. However, this is given that the colder temperatures do not cause any health problems, and for fancy goldfish, they almost always do. The only case where lowering the temperature might actually extend the duration of a goldfish’s life is for a common or skinny-bodied goldfish. They can withstand the cold with no harmful long-term effects, but a fat-bodied fish will likely get sick in colder temperatures and end up having a shorter life due to more frequent illnesses and stress. However even for common goldfish, I believe it's better to have a shorter lifespan with less risk of disease and sickness than a longer lifespan with a slightly greater risk of disease and sickness, but they are hardy fish so the choice is yours. 
I personally try to keep my goldfish at roughly 78F (25.5C), I do this for several reasons. Most of my goldfish are fancy goldfish so they do require elevated temperatures, as a keeper, I have the goal to grow my fish large and fast (this is just a preference of mine) and elevated temperatures help with that, and I want the immune system of my fish to always be at peak performance and I believe this is the ideal temperature for that. Fancy goldfish also love to develop swim bladder disorders, and I have found higher temperatures are the best way to prevent that. At this temperature, I believe the water is just warm enough for all the positive benefits of increased water temperature, with an acceptable or a low amount of the downsides like lower oxygen levels and stress in the fish. Every fish is different, and this temperature might not be ideal for some people’s stock, but it is what works well for mine. 
Everything written in this article is from a combination of my own past experiences, past research, talking with other keepers and hobbyists, and general knowledge. I ALWAYS recommend doing further research before doing anything to your own goldfish.
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