You know, there actually isn't any depiction in the games themselves of what exactly the E-tank liquid looks like. I based the glowing, green colour off how it looked in the Ruby-Spears Megaman cartoon. The glow probably comes from the nanobots inside, and it's how you can tell whether or not it's still good to drink. I guess that means E-tanks have expiration dates. Though, considering how you can still use them in Megaman ZX and ZX advent, they either don't go off for hundreds of years, or the infrastructure and techniques to make them survived the several world-destroying cataclysms and centuries of war. Pretty Impressive, either way.