some goldfish can get a lot bigger than a lot of tropicals too... i assume a lot of aquatic staff are told to tell customers to keep a minimal amount of goldfish per whatever the tank size is. certainly better than overstocking and having the tank crash. alan is right, if all conditions were the same (filtration, water changes, plants, lights, tank size) then technically you can stock more of the same-sized fish (that create a similar amount of waste) as cooler water contains more oxygen than warmer water