Is a 17 liter tank big enough for 2 goldfish?

A 17 liter (4.5 gallon) tank is not big enough for 2 goldfish. Even very small fancy goldfish should have a minimum of 20 gallons (75 liters) each, and single-tailed varieties such as comets and shubunkins need even more space. Goldfish are very messy fish, and they produce a lot of waste, so a small tank will quickly become dirty and harmful to their health. In addition, goldfish are active swimmers and need plenty of room to move around. A 17 liter tank would be too cramped and would stress the fish, which would shorten their lifespan. If you are interested in keeping goldfish, you should get a tank that is at least 20 gallons (75 liters) and be prepared to provide regular water changes and proper filtration to keep the water quality high.