Contact lenses are stored in a purpose-built solution that is recommended by an eye care specialist. Contact lens wearers should stick to the recommended solution as the recommendation will be based upon the wearer's eyes and medical history. Water should never be used, again due to the fact that it can increase the risk of infection.
Old solution in a lens case should never be "topped off" with new solution - instead, only fresh solution should be used. Solution can become contaminated with microbes that cause infections, and simply adding fresh solution to old solution reduces how effective it is at eradicating germs.