Mafia Wars, Fighting at Certain Levels

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The higher in level, the more difference in the levels you see in the fighting list.

At level 300 or 301 you will be in the mini-shark tank and you will see players between level 301 and 450. When you are over level 450, you are in the shark-tank and you'll see people of level 2000+ in your fight list (451 and everything above).
The levels in one fighting page are similar. In the mini-shark tank, you might see a page where everyone is between 303 and 305 and in another city they can be between level 420-422.