List of Sub Tank locations

Here is the list depicting where the various Subtanks (and other related items) in the Mega Man X series onwards are located.

Mega Man X

 * Storm Eagle: In the first part of the area where there are platforms rising up and down. Use the platforms to reach a tower to the left. The subtank is inside the tower; shatter the glass to get inside.
 * Spark Mandrill: At the beginning of the stage, there are two paths, one leading up and another down, then forward. Follow the latter path to a dead end, and use an aimed Boomerang Cutter to nab the subtank on the other side of the wall.
 * Flame Mammoth: Found in the same large room the Heart Tank was found. Make your way to the top of the platforms and head to the left past some Mettools. At the end is an extra life, and you'll find the Sub-Tank if you jump from here and climb up the left wall. Use the armor's foot upgrade to break through the blocks.
 * Armored Armadillo:After dropping down into the mines, a drill-like enemy with spikes will break through a rock wall and chase after you. Hang around on the wall and allow it to pass you, and explore the cubby where the tank was to find the Sub-Tank.

Mega Man X2

 * Flame Stag: In the very beginning of the stage, there's a large hornet robot that you're supposed to trick into ramming the breakable blocks. Instead, lure it up to the top of the mountain, and use its back as a platform to jump to the otherwise-unreachable cliff on the left. The Sub-Tank is sitting against the wall on a ledge.
 * Bubble Crab: Following the area where the heart tank is, drop down into a similar section of the stage with more robotic jellyfish. Using a charged Bubble Splash, which allows you to jump higher underwater, jump up to a small spot in the middle of this area that's above water. X will just break the surface, and you can actually jump on the surface if you press the B-Button as you're about to land. With this in mind, the Sub-Tank is on a ledge to your left.
 * Magna Centipede: After the puzzles involving the large blocks that move around and try to crush you, you'll see a small gap in the ceiling with a bit that sticks out, begging to be climbed up. Stand on the raised platform just off screen to the left and perform an air-dash coupled with a charged Speed Burner to reach it.
 * Wire Sponge: After passing the first weather control orb, stand on the first platform and air-dash to the left wall. Follow this up and stay on the highest set of platforms to find the Sub-Tank on the last one.

Mega Man X3

 * Neon Tiger: Near the beginning of the stage, there's an open area with the insects that you can stand on. Though you can use them to bypass the second level, climb up the right wall and destroy the red robot that swings a mace at you. The Sub-Tank is directly above you in the far right corner, so use the platform-insect to reach it. Alternatively, you can also use the enhanced boots and perform air-dash up to it.
 * Blizzard Buffalo: This subtank is in plain sight above you as soon as you enter the last open area before the boss. Use the foot parts' (conveniently found in another part of the area) upward dash to reach the subtank.
 * Tunnel Rhino: In the part of the tunnel where columns of dirt are dropped on you and the moving platforms, the Sub-Tank is in the far right corner. You're supposed to drop down a series of steps, and you'll probably spot it as you're going. Simply jump to the wall and climb up to obtain it.
 * Volt Catfish: Beyond the room where you fight Bit or Byte, there's a long vertical passage with orbs of electricity running up and down. Head up top first and jump into a Ride Armor when you find the Ride Armor platform. Now return to the vertical tunnel and jump down. When you reach the bottom, the Ride Armor's mass will cause the floor to crumble beneath you. Follow this new path to a hidden room where you'll find the Sub-Tank and some energy capsules.

Mega Man X4

 * Jet Stingray: Area 2 above some platforms.
 * Cyber Peacock: achieve Rank S during the second "test" (as X) or third "test" (as Zero) of Area 1.
 * Weapon Tank: in Area 2 of Frost Walrus' stage, at the top in the last column of cube of ice.
 * EX Tank: in Area 1, also in Frost Walrus' stage, above the disappearing slopes with spikes. Climb the wall to the right, and use Lightning Web to reach the tank on top.

Mega Man X5

 * Grizzly Slash: the last portion of the exploding trucks.
 * Dark Dizzy: At the very end of first part of the stage, (the part with the constellations) there will be a platform with the subtank on it. The problem is that it's too high! Use the Falcon Armor to fly up there. If you don't have that, you can still reach it simply by jumping from the moving platform as it gets there. As Zero, you can also get it by using E-Blade, or if you have the C-Sword, you can just double jump.


 * Weapon Tank: The Skiver; in the area with the large descending elevator. Beat the elevator to the bottom, where a crumbling platform will be holding the tank.
 * EX Tank: Izzy Glow; in plain sight in the hallway with rising and falling spiked platforms; aim a Ground Fire on the wooden plank(s) above you that are blocking the way to the tank.

Mega Man X6

 * Blaze Heatnix: in the Nightmare Area.
 * Commander Yammark: You'll need the Blade Armor. After you drop down the first time, keep heading left. Eventually, you'll see a ledge up above you. Air-dash up there, then do a long air-dash to the left. There will be an Injured Reploid and the Sub Tank on a small ledge.
 * Weapon Tank: Shield Sheldon; lying on a floor of spikes in the Nightmare Area. Requires the Shadow Armor; alternately, just obtain temporary invincibility by getting hit by an enemy.
 * EX Tank: Blizzard Wolfang; requires Shadow Armor. A few steps forward after the heart tank, there will be a vertical shaft lined with spikes. Drop down, and head forward to find the EX tank.

Mega Man X7

 * Splash Warfly: Look for the Sub-Tank behind some crates.
 * Vanishing Gungaroo: At the end of the stage, when you fight against a squadron of Ride Armor Reploids, there are several columns. Climb one of them. Up here, you will find several items, including a Sub-Tank.  Try to do this when you have one or two Ride Armors to destroy, but before you destroy them all (which triggers the boss battle).
 * Weapon Tank: Ride Boarski; located behind a silver crate.
 * EX Tank:Soldier Stonekong; right before entering the boss room, stand on one of the "Stonekong heads" and ride it upwards. Then, double-jump into the trees on the left.

Mega Man X8

 * Subtank
 * Optic Sunflower: obtain 5-7 S ranks in the stage.
 * Dark Mantis: get hit by an enemy searchlight, then use Axl's Copy Shot to obtain a Guard enemy's DNA. At the far left, there will be a hidden room, with a guard. use Axl's obtained DNA to transform and salute to the guard, to open the way to the generator room. Use a charged Crystal Wall on the triangle on the floor next to the generator.
 * Earthrock Trilobyte: while being chased by the large Mechaniloid, there will be a bone structure. Jump onto the structure and dash forward to see the Metal in mid-air.
 * Weapon Tank
 * Burn Rooster: in the beginning of the second descending scroll (the one before the boss), jump up instead of going down. There will be a hidden room above where you entered the area.
 * Intro Stage: once the player reaches the first waterfall, jump upwards (use Zero). The Metal will be on the ledge.

Zero Series
With the exception of Zero 4, most subtanks in each game are obtained from Cyber-elves.

Mega Man Zero 2

 * Hyleg Ourobockle: before the falling spiked ceiling. Chainrod skills are needed to obtain the subtank.

Mega Man Zero 4

 * Noble Mandrago: After the mini-boss, at the top of a spike-lined chamber.
 * Fenri Lunaedge: Use the Zero Knuckle to acquire a Card Key from a crab enemy; this enemy can only be encountered in the lower route, where entry equires you to melt some snow. After the mini-boss, use it on the control panel to the right of the electrified door.
 * The remaining two are obtained from Hirondelle in the Resistance Trailer in exchange for two S-crystals.