Liveaquaria's recommendations are generally accurate (if sometimes on the larger side), but gallons alone should not be relied upon. The length of the tank, the aquascape, tank mates, and filtration capacity also play a role in determining whether a fish is suitable. As a rule of thumb, fish generally would prefer the front of the tank to be around 8 times their length, and the sides to be 4 times their length. As such, a 6 inch tang would do best in a tank of at least 48x24 inches in footprint, and a 2 inch damsel would do best in a tank of 16x8 inches. Of course, some more sedentary fish (gobies, blennies, frogfish, clowns, etc) can go in smaller tanks, and these rules are more of guidelines. I hope that this helps!