On the two extra Hard+ levels, there appears to be almost no fuel at all. However, on level 7, there is a slightly hidden fuel dump: after you shoot the button to open the first door to gain entry below the planet's surface, you must wait for it to close and the second door below it to open to venture further. After that second door opens, shoot the button again (while getting out of the way before it closes on you), then fly down and to the right through the now open corridor (passing the vertical corridor that houses the guns, pod and reactor). This will lead to three fuel cells, along with a button to open the door back up again.
If the player can make it to the reverse gravity planets, once the game ends, a third option will appear on the main menu.
Pressing button 3 on the controller will take the player to ZSB, the hidden bonus game.