CCWC1988Legionnaires should not be a problem in a static tank of pure water, and why the ref to filters. Legionnaires is generally found in apparatus that are not regularly cleaned, where the water is warm, and where the water is vaporise is some way to form tiny droplets that are carried in the air and are then breathe in by people. Such as its original source in an air conditioning cooling tower in the USA where at a meeting of the " American Legion” in 1976 members were unlucky enough to inhale the vapour, hence the name. Apart from cooling and air conditioning units the only other outbreak I have heard of was in a large jacuzzi/spa on a cruse liner, again poor disinfecting, warm water and fine droplets of water from the jets. If any one has any info on cases of legionnaires in pure tanks of water please let me know. If any one is really worried then dose the water with 5ppm chlorine or fit a small UV with a re-circulating pump.More info at http://www.hse.gov.uk/legionnaires/steve@freshwatersystems.co.uk