Tank 6 is a glitch tank that can be unlocked by fighting Cyrax multiple times in a row. To do that get to Tank 5 and defeat Cyrax normally but don't enter Tank 1-1 afterward, instead click on the user panel, double click on your name and enter Tank 5-1. Continue with the Tanks 5-2 – 5-5 until Tank 6-1 appears on the Adventure Mode. Click on that and you'll notice that it's like an infinite void cheat was activated and that the fish leave streaks of themselves when they move. Also once here there's no way out of it and unless you have the Fishinator 2 you will have to start a new game and lose all your data. However the Cyrax glitch can net you a ton of shells. Be sure to stop after Tank 5-4 to avoid locking yourself into Tank 6. The no man's land is in short what Tank 6 simply is.