I used to hate goldfish because they were always boring gold, they got huge, and they make a big mess. I have never owned a goldfish. Now I have discovered black moor and shubunkin goldfish. I am planning on getting one in the near future. I only want one, at the very most two. I have a 20-gallon fish tank that will be available for me to use when the three fish that are currently in it die. I have no idea when that will be, but when they are gone I can put a goldfish in it. It has a sponge filter rated for a 35-gallon tank in it, and if needed I could also put in a Tetra Whisper 10-30I filter. Could I keep one black moor and/or one shubunkin in it? If I bought a 30-40 gallon tank, how many goldfish could I put in it? So basically my main question is, what is the smallest/cheapest set up for one or two goldfish?