10 gallon "leader."

I would think it is called a leader because it is the most common.
 
Sales leader, loss leader...etc. I'm sure they sell WAY more 10G's in the US than any other size, probably with 20H or 55G second. No "magic" to the name...
 
If the answer is really that important to you, why not contact the manufacturer of the tank and ask them? Who IS the manufacturer anyway, All Glass, Perfecto, Top Fin? Someone else?

A question like this is like asking "Why did Chevrolet decide to call their two door sports coupe a Camero"? The name means nothing in any language, and certainly nothing in the language of their largest market.

The answer is "Because someone at the company decided that they liked the way it sounded, and that it would attract customers".

The answer is the same for your tank. "Leader" implies market dominance, and also that the products of other manufacturers are "followers". If a product really does dominate the market, there is often a good reason, perhaps it really is better than the competition, or at least a better value, ideas which might influence someone in the market to purchase a new tank.
 
The problem with that answer is that:
a. "Camaro" is a nonsense word but "leader" is a word that has a meaning in the English language.
b. All of the other words used to describe the tanks are words indicating either their intended use, or a description of their shape. You have tall, long, breeder, etc. The words aren't being used in a way that is pleasing to the ear or trying to attract buyers. They are merely descriptions of the tanks.

Companies don't often name their products for how well they sell compared to other models. You don't see Toyota calling the Camry a "Leader." So what's the point of calling a ten gallon tank the same?
 
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