When using contact lenses, you must use the specified lens solution. Sometimes, tap water is used because the solution looks similar to normal tap water.
Tap water contains a parasite called a spiny amoeba and several bacteria. This is not a problem when eating or using it, but if it enters the cornea, it can cause eye infection and cause loss of vision. In severe cases, it may cause loss of vision.
Even if the lens is not stored, some people wash it off with tap water for a while and put it on. This action can also adversely affect the health of the eye, so it is wise to use artificial tears rather than tap water.