Travel to the main elevator leading into the green jungle region of Brinstar and head through the first door down the shaft on the right. Jump up to the
upper platform and use the Speed Booster to run past all the closing
shutters. Head through the door on the right to find this Reserve Tank
(there's also two missiles hidden in this room).
In the large green room to the far right of the main elevator past all
the fiery caverns, use the Grappling Beam to hook onto the blocks at
the top of the room, then swing over to the green door on the left. Blow
through the door with a Super Missile, head in and grab the missile on
the pillar. Before you go, jump over to the wall on the left and fire
straight down to reveal a hidden pillar that will begin to rise.
Once
it stops bomb open the wall on the left and head through the hidden
tunnel to find another door. Go through and traverse the lava-filled
cavern. Once you reach the end of the corridor you'll spot the Reserve
Tank sitting atop a ledge (don't forget the other missile hidden here too).
Once you find the Gravity Suit go back into the large corridor
where the Chozo statue above brought you down to. Blow open the floor below
the door and travel right through the hidden tunnel. You'll eventually
discover another hallway leading off to the right, move through and knock all the
walking robots into the spike-lined pits.
You'll see a missile just ahead,
grab it and lay a Power Bomb. The blast will destroy the statue and
open up another hallway, once it's open run back to the left side of
the hallway. Run back to the right, then once the Speed Booster kicks
in crouch and quickly walk over to the end of the hallway past the
conveyor belt. Jump straight up and you'll soar up into a new room. Go left and
descend down through the floor, you should now spot the Reserve Tank dead
ahead.
Be careful here, use the X-Ray Scope to find a hole in the
ground right in front of your prize, then jump over it to reach the tank. Descend through the hole once you've secured it to escape back to the lower hallway.
In the giant room with the huge vertical pipe that transits you between
the upper and lower levels of Maridia, go to the bottom right-hand
corner of the room and bomb open a hole in the floor to reach the
sandpits below. Cross over the first pit and sink into the one on the
far left. Descend through the sand-filled shaft into a larger cavern below.
You'll find the Reserve Tank along with a missile at the top of the
room on the left side, but it'll be handy to have the Spring Ball and Space
Jump to reach them easily.