Here is my little informative thread about single tailed goldfish, in particular common's, because that's what I have most experience with. From what I've found looking around, is that while single tailed fish do okay in aquaria as young or juveniles, ultimately they need to eventually end up in a large pond or a very large tank.
When I say large, I'm talking a 1000 litre pond or 6 foot tank.
Why 6 foot?
Well we all know about stocking rules, but one interesting rule I have come across, is that a tank should be, ideally, 6x the length of the fish. So when you consider single tailed goldfish have the potential to grow to 1 foot in length, using the rule as a guide it is clear that 6 foot is the minimum size for an adult fish.
Some of you may agree with the 6x fish length rule, some of you may not agree, I think it depends on what type of fish you are keeping. I personally think it applies to common goldfish well, because they grow so long and as a guide, it helps us pick ponds they can have lots of swimming space in.
Now the difference with ponds and tanks, is that a 6 foot tank will at most (from what I've seen) give you around 500 litres. But I have seen a pond tub, that is 6 foot x 4 foot x 2 foot and holds 1300 litres of water.
So in my mind, a pond is the way to go, the tubs are often cheaper than any tank and hold twice the volume.
This is the pond tub, I refer too, just in case you were wondering, as until recently I didn't know what they were;
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Of course not everyone can jump up and start digging a hole in their garden to line or build a pond using some preformed plastic, as you may not even have a garden and a pond has it's own requirements, such as getting electricity outside or stopping predators getting at your fish.
You could always consider having an indoor pond, if you have a spare room inside and not much garden space.
In some situations it is appropriate to re-home the fish.
Again preferably a pond, perhaps someone in your neighbourhood has a pond that could take the fish or a local garden centre has a display pond where you know the fish won't be sold, or a local park that has a pond, or in some cases fish shops can take the fish, or there may even be a local 'fish rescue' service.
If you are un able to re-home your fish, large tanks (as mentioned above) can be an option, you could possibly keep a single tailed fish in a smaller tank of 3 foot or 4 foot and upgrade as they grow. Using the 6x fish length rule to help, so for example a juvenile fish can grow up to 6 inches in a 3 foot tank.
The reason I suggest finding a pond home first, is because in my opinion a pond is better than a tank.
If you are able to build a pond or un able to find a pond home and decide to go for a large tank, please consider the commitment each one needs, for example on a tank you will need to change at least 50% of the water weekly (on a 500 litre tank, that is 250 litres weekly), with a pond this is 10-15% weekly (on a 1300 litre pond that is around 130 litres), but you also need to top up the pond water to account for evaporation as well as the changes. Not to mention there is also feeding and cleaning, as well as filters to consider. So please research first.
Now some people reading this, will think I'm crazy, why does a fish need so much room?!
I understand I thought the same thing when I first started, so they say pictures speak a thousand words, here's a very cute juvenile goldfish;
You will eventually, have a fish like this;
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Also illustrates things very well, it is a tale of four goldfish and covers how tank size and living conditions can affect the fish.
Now I would just like to add, that this is information I've found looking around other forums that I have agreed with, being that single tailed fish belong in a pond. There are many, many different views out there, with regards to goldfish.
Myself, I currently have a juvenile common goldfish and I did everything wrong.
My fish doesn't have a 6 foot tank or a pond. In fact to begin with, I had two of them in an un-cycled 21 litre tank.
It was completely wrong, I lost one fish within a week and have since been doing my best for little Lucky. She is a 3 inch juvenile goldfish and currently lives in a 3 foot tank (photo below).
Now I'm using the 6x fish length rule, as a guide for when she needs to be upgraded to either a bigger tank or a pond.
So with the tank being 3 foot in length, she should be okay in there until she is about 5 or 6 inches.
I'm not sure if I can provide a 6 foot tank or build a pond, so I am considering the option of re homing her this summer or next summer, depending on her growth and health.
Now I know I have missed a lot of information here with regards to stocking a pond and tank, this is because I would personally have a few fish, in a large space. I personally feel that if I had a 500 litre tank, I would have 3 or 4 goldfish and in a 1300 litre pond, I would probably only have a shoal of 6 or so.
I wanted to share what I've found, not to make people feel bad, or to anger people, but I just wanted to show that at the end of the day there are lots of options, for us to choose from, so that we can give our fish the best care and the best home
And lastly, if you haven't brought any fish yet and are considering the common or any single tailed fish, unless you can provide them with a pond, then please avoid them altogether!
Instead I would advise considering one of the beautiful fantails or another fancy variety of goldfish.