This tank is not suitable for goldfish; 15 litres is far too small for even one goldfish as bought from the store, and the statement above that a couple of goldfish could live in here is not only inaccurate, but unethical.
One goldfish as a juvenile (when you buy them from the store at around 6 months to a year old) needs 10 gallons - i.e. 45 litres, 3x the size of this tank - and that won't even do for long. Later, your goldfish will come to need at least 30 gallons to survive. A small tank like this will allow toxins to build up and too little oxygen will be present in the water - eventually, your fish will die from stunting when pressure on the internal organs becomes too high (assuming it doesn't suffocate or become poisoned first). You will have an inactive, ill pet for a few months. Your goldfish should rightfully live for over a decade; depending on the type, maybe even two. Tanks like this contribute to the myth that it's an achievement to get a goldfish to live five years - that's the equivalent of a human dying in his/her thirties.
One day (hopefully) the manufacturers will just stop making these deathtraps, but until then we need to vote with our wallets and buy healthy, safe tanks for our pets. Thank you for reading, and please don't buy this tank.