How do I replace the low or discolored water in my snow globes? ?

I love snow globes and have bought several unique ones on ebay, picked them up at garage sales, even at Goodwill. Why does the water discolor? Some even have scum in the water or the water level has decreased. If they aren't leaking how does the water get out of them? Do I replace the water with distilled water and should I reuse the glitter, snow, or sleet that is inside them? Thanks for your help.