Mega Man X/Heart Tanks

Heart Tanks increase X's maximum health by 2. There is one tank in each maverick level; thus the maximum amount of health X can have in the game is 32.

Chill Penguin
After beating Flame Mammoth, return to this level (if you've beaten it already) and when you get to the part with the Mech Armor get in it and ride it to the part where there's a cave you have to go through. At the left entrance you can jump off the ledge straight up, then jump out of the Mech and up onto the ledge. Use Fire Wave to destroy the first misquito bot generator building up there and you'll see the Heart Tank lying inside.

Alternatively, you can go through the cave and then jump up on top from the right side by dash jumping across the small gap.

Flame Mammoth
After beating Chill Penguin, follow the level until you run into the pick axe throwers. From there, run across the ice covered floor to the very bottom right. The Heart Tank lies in the corner.