If you're looking at a potentially 8-10" (and very round) fish, I would want to keep it in something larger than a 30gal. JMHO.
if you read my post. you will see, that mine were about 4 inches and the waste was way too much for a 30l at their size and i had a mag 350 and a 30-60 hob filter also,.. but still wasn't good enoughi dont see why 30gallon long would be ok for just 1, i have 2 fancies in my 30long,and they are growing well, they are overfiltered and have weekly 60-70% water changes, but are happy and healthy.
I can't imagine a common goldfish being particularly healthy or happy in a 2 gallon bowl...as they get up to 10-12" with proper care and accommodations. If it is not growing, its stunted and unhealthy, so I wouldn't use that as a baseline for comparison.