Sub Tanks are found in 4 stages in all 3 of the games and they are pretty much an extra life without dying if you fill them to the top.
Flame Mammoth's Stage:
In the room with the Heart Tank, go to the top. Once there, go all the way
left, and use a combination of the leg and head upgrades to destroy the
blocks blocking the sub tank.
Spark Mandrill's Stage:
When you reach the first "down-ladder," go right (you should be at the bottom
right of the room.) Use the boomerang to pick up the sub tank.
Storm Eagle's Stage:
When you reach the area with platforms with fire turrets on them, get to the
top, and go in to the glass building on your left. Here you will find the sub tank.
Armored Armadillo's Stage:
When you get to the first Spike-Tank, wait for it to pass, and enter the room where it came out of to pick up the sub tank.
Wire Sponge's Stage:
When you get to the part with platforms over spikes, get on the first, and
jump over to the tree. Climb it, and go right. Up here you'll find the sub tank.
Flame Stag's Stage:
On the first beetle that you see, jump on its back. It'll carry you way up.
Jump on the left platform to get the sub tank.
Bubble Crab's Stage:
After the horizontal platform that the fish opens, go right a bit to one of the platforms a bit in. Charge up the Bubble Splash so you can jump to the top of the water and jump again to get on the platform with the sub tank.
Magna Centipede's Stage:
You have to set off no alarms the same way as the heart tank. When you reach the last section where blocks fall from the ceiling, quickly get past them (before they all fall) and jump on the lone block that comes from the ceiling nearby. Ride it until it's over the gap, then dash jump to the wall, climb up to get the tank.
Blizzard Buffalo's Stage
As soon as you enter the snowy area, you will see the Sub-Tank on a ledge.
Use the air dash enhancement to reach the ledge.
Neon Tiger's Stage:
You'll find this item very early in the stage. When you enter the first
large room, go to the upper-right corner. Use the flying bug platform to reach
the tank.
Tunnel Rhino's Stage:
Early in the stage, you will see units that spew out sand. After
that section, you'll have to go down. The Sub Tank is around right before
you head down.
Volt Catfish's Stage:
One point in the stage there is a lot of electricity moving around in the
background. Head up and go to the left to find a robot ride platform. Choose
any armor and fall back down. The floor will break, and you can procede to the
left to find the Sub-Tank.
