Filters can present a first, second and even third line of defense against potable water contamination. When filling tanks, running dockside water through a high-volume inline filter will ensure clean water is brought aboard. (Note: Dockside hoses are notorious for lacking a potable water approval rating.) However, filters containing activated charcoal should not be used for this purpose, as they will neutralize the chlorine contained in municipal water supplies, which will likely allow biological growth within the tank and plumbing. Also, liveaboard sailors who plan on leaving filters on the dock should make sure they have opaque canisters, as clear canisters are ideal breeding grounds for algae.