greetings..

im interested to keep goldfish and have very little knowledge about it.. i really appreciate ifanyone can advise.

1) i have 1.5 ft tank (10 gallon/40 litre) for 2 goldfish (size around 5 cm from head to tail).. is it enough? i read that 1 goldfish need at least 20 gallons, is there any reason for this?
2) do i need to put a little salt ?
3) what food do u always feed ur goldfish
4) how many times that i need to change my water (using RESUN biostream filter)? should i change only 1/3 of the water?

pardon for my bad english.. any advise will b much appreciated