Wire Sponge Stage
When you see a flying platform, step to the first one at very top and then dash jump to the left, then jump right to the next platform and you'll find the sub-tank here.
Flame Stag Stage
At the beginning of the level find the first flying beetle. Jump on it's back and it will rise upwards. Let it rise as far as it will go. when you reach as far as it will go, you'll see a ledge on your left. Simply jump to it and find the Sub Tank.
Magna Centipede Stage
Right before to the mini-boss room, there is a gap in the ceiling. If you are unsure of exactly where this is, utilize the Helmet Upgrade. Make sure when the multiple boxes fall to quickly pass over it, and then a singular box will fall, ride it and quickly dash jump to the right to get a sub-tank. Alternately, if you have the X-buster Upgrade, and the Speed Burner, you can just dash jump across the pit that precedes the gap in the ceiling, and at the height of your jump release a fully charged Speed Burner to dash into wall that leads to the gap.
Bubble Crab Stage
In the open water area, after the fish opens the big red door in the ground, there is a flat surface that stretches out for longer than most. the fish bot will glide very close to the ground here. Charge up the bubble weapon so you can jump super high. When you jump you'll see it on the left. When at the surface tap the jump key again to the left.