Fancy goldfish, or twin tailed varieties, do not grow to be nearly as large as their single tailed varieties and make much better pets. However, fancy goldfish produce almost ten times the amount of waste similar sized fish do. This means that goldfish need a very large amount of water to dilute their own waste or it will build up to toxic levels in the tank. Essentially, a tank that is too small could poison the fish and cause the fish to become sick or even die. The one thing that all successful goldfish keepers agree on is that the larger the tank, the healthier your fish will be. Please educate yourself on the nitrogen cycle before reading the rest of this article.
The minimum tank size for fancy goldfish varieties is at least 20 Gallons or about 75 liters, anything smaller could lead to unstable water conditions unless the water is changed daily. Most varieties of fancy goldfish will require 10-20 gallons per a fish, preferably 20 gallons per a fish. However, some varieties like orandas can grow larger and will require a minimum of 20-30 gallons per a fish. That said, these are the minimum and people typically have healthier or longer lived fish when they are provided with even larger tanks. A serious mistake made by beginners is believing that just 1 more fish won't effect the tank, only to find sick or dead fish.
Goldfish naturally produce a hormone that will signal the fish to stop growing if it begins to build up in the water. This is why goldfish in bowls never grow and only live for a few months. The less of the hormone there is in the water, or the bigger the tank, the faster and larger your fish will grow.
Fancy goldfish will be fine with other fancy goldfish. However, if you want a big community tank then, goldfish are not for you. Goldfish will produce a massive amount of waste as mentioned earlier that may kill your other fish.
The shape of the aquarium is very important. Goldfish swim from side to side, not up and down. A long shallow tank is very well suited for goldfish. A tall deep tank like the one shown in the picture is not suitable for goldfish and could be very stressful to the fish. There are a number of interestingly shaped aquariums that may be tempting, but none of them are a good fit for goldfish. Stick to the basic rectangular design.