What he's trying to say is it doesn't always work because not all fish are compatible because of issues like aggression, diet, or other things. For instance you can't keep 2 neon tetras in a 2 gallon tank because they need groups and need more room. Also you can't keep two 5 inch fish in a 10. There are always more factors than you see just by inches. A good example would be that I have 13 inches in my 10 (a beta, 9embertetras, and an otto) but consider it stocked right whereas 2 yellow labs (4 inches) jn the same tank wouldn't work because of aggression/size and 5 black skirt tetras wouldn't work because they need more room than a 10 gallon offers.