Goldfish need 10 gallons per goldfish no matter what type. You don't need a filter, but it's a good idea so the ammonia levels don't spike every time you do a full water change. I'm thinking if you have no filter you will dump all the water out and then the ammonia levels will be constantly going up and down. This will put a lot of stress on a fish and goldfish aren't know for dealing with stress well. Plus, if the ammonia goes too high it will poison the fish and it will die. A single goldfish should do well in a 10 gallon (minimum) cycled tank with a filter and 25% water changes weekly. Fish, of any kind, are the most complicated animals to own, in my opinion. I have cichlids at the moment and it gets really expensive.