Vendors are always going to err on the side of caution and over estimate.
Compared to other sites, this one does recommend much more liberal stocking. Think of it this way:
This site is going to attract those with smaller aquariums, so naturally other sites won't be as appealing to those with small aquariums. As a result everyone who uses them migrates here, and whats left are those who are traditional and suggest the ridiculous things like "1 inch/ten gallons" and other stupid rules of thumb. These same people then put 5-6 tangs in a 180 gallon tank, breaking their own "rule."
I think this whole gallons definition is stupid also, because the footprint of the tank is much more important than the number of gallons (assuming the biological load isn't the issue). Fish tend to swim back and forth along a tank, not up and down.
If you're looking for a fish, I would stay away from the six line, because that seems like it's pushing it more than the firefish or other fish on that list.
Edited by asting, 03 March 2012 - 07:57 AM.