Originally Posted by Tikibirds
I see this around alot - why does the 1st one need 20G and the 2nd only need an extra 10g? If the 1st one requires 20, shouldn't the 2nd one also require 20 gallons making it 40 for 2??
It's more of a guideline than a rule. Surface area is another large think that people need to take into account. A 30 gal hex is a very different tank than a 30 gal breeder. A 30 hex couldn't fit one goldfish let alone the two that this rule claims. What it comes down to is that this is an attempt to keep people from overstocking a goldfish tank. I'm sure that some serious goldfish enthusiasts could keep 1 goldfish per 10 gallons, but they would also have the money to dedicate to a major filtration setup. The average person doesn't have the ability to invest in that commercial-grade filtration, so that is why we add that extra 10 gallons in there. It's a buffer zone.