Yes. Do not leave tap water or any other kind of water inside any aluminum or aluminum-lined cylinder—or any steel cylinder, for that matter. Water remaining inside a cylinder is considered a “contaminant.” Luxfer, as well as regulatory authorities around the world, have required for years that cylinders be kept free of contaminants. If a cylinder becomes contaminated with tap water, pure water, sea water, oil, grease or other contaminating substances, the cylinder should be removed from service, properly cleaned, thoroughly dried and diligently inspected before further use. However, in some countries, regulations or standards require that cylinders containing water be condemned and rendered unserviceable (for example, see ISO 10461, Section 14.2). In short, if you find water or other contaminants inside your cylinder, take it out of service and comply with all applicable regulations.
As far as 6061 aluminum alloy is concerned, cylinders made from this alloy have been used for decades in scuba and shipboard service, in which cylinders are periodically exposed to very corrosive salt water and brine. These cylinders have an outstanding record both for service and safety. However, at no time should cylinders be stored sitting or standing in water—they should be stored in a clean, dry place—and water should never be allowed to remain inside cylinders. The only time that cylinders should contain water is during the very brief period required to perform a hydrostatic test (see questions 2 and 3, below), and the cylinders should be immediately dried when the test is completed.