How much water do goldfish need?
If you’ve done your homework and done a little research before choosing a tank for your goldfish, chances are you are probably now more confused and frustrated than before. Different sources give different and often incompatible answers… so who should you listen to?
You will likely have been told that each goldfish needs 10 gallons (if you live in the US) but strangely enough only 20 litres (1 litre = 0.3 gallon) per fish if you live in Europe; alternatively you could have been told to calculate 5 litres for each cm of length of the fish, or consider 40 litres for the first goldfish but less for any additional fish. As a matter of fact, you are often told any quantity ranging from 5 US gallons to 50 gallons per fish. So, which answer is true?
All and none.
There are only two rules you can safely take for granted when choosing the right size of your tank:
1) a bowl is always inadequate (insufficient volume compared to fish size; insufficient surface area for gas exchange; no room for a filter; impossibility to keep steady water parameters, etc etc)
2) the larger the tank, the better (goldfish are active swimmers and can grow to a considerable size; the largest the volume of water, the steadier water parameters are, etc)
But aside from these basic principles, there is no such a thing as a universal golden rule, and it’s only experience and good sense that will help you choose the right tank size for your goldfish.
Think about it. Goldies come in a wide range of varieties, from slender swimmers such as Shubunkins and Comets to the clumsier Pearscales and Ranchus, and usually range from approximately 3 cm / 1 inch to 20 cm / 8 inches. When you think about such diversity, you realize there can’t be such a thing as a universal rule.
The “litres-per-cm” rule may deceptively give you the impression that it can be universally applied, but it is just as relative as the others: in fact the litres-per-cm rule can be especially misleading if you have very small fish because it can induce you to think that a very small goldie can live in a 20 litre tank. However such a small amount of water is inadequate to set up even a simple aquarium. If you want to have a chance to keep steady water parameters you should consider a tank no smaller than 50 litres. Said so, the litres-per-cm rule also fails to point out another important factor: it is advisable to keep no less than two goldies together. While a lonely goldie will survive, goldfish are by nature social animals and will benefit from the company of another member of their species. (In countries such as Switzerland, where I live, it is explicitly forbidden by law to keep less than two goldfish in the same tank).
When choosing the size of your tank you should keep in mind the quantity and quality of fish you are going to keep, as well as your future plans. Are you planning on keeping the goldies in that tank also when they grow up? (Bear in mind that if your goldies do grow up and develop their full potential, chances are as adults the only tank suitable for them will be actually a pond!)
Or do you have serious intentions to upgrade it later and move the fish to a larger tank? (“Serious” means you have the well-being of your fish in mind and that you won’t wait until the old tank is too small for them to set up a larger one!)
Whichever choice you make is a personal matter, and you should ponder it honestly and carefully.
Many fish keepers will advise you to purchase the largest tank you can and stick to it, and it’s good advice indeed.
Personally I decided that I’d rather set up different tanks in the course of the years, in order to better suit the needs of my growing fish at any given time. This way I am given a chance to experiment a little with different plants and kinds of environment, in order to provide my fish with the one that better suits their needs. (I must say I’m more concerned with finding the ideal environment for my fish than creating a marvelous piece of furniture. ;) )
However this amounts to loads of work. If you want my advice to keep things simple, I suggest that as a starter you buy a 120-150 litre (30-40 gallon) tank for two young fancy goldfish. This will provide them with a suitable environment for a long while. Keep live plants in it and learn to observe your fish – their behaviour, they growth, etc. (Measure them if necessary and keep track of it.) As months pass by you will get to know your fish and their needs, and that will give you a better idea of any future changes you may need to make (including providing them with a larger tank sometime in the future) than any advice a complete stranger can possibly give to you! :)