Water saving tank alongside mains

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I a house with low pressure water a Monsoon Pumb can be used to increase water flow at taps on demand is there a similar method I can setup for a large rainwater tank with the extra complication of a mains water connection if the water butt is empty! I can place the tank in a shed for protection of the electric and I basically want to use the rainwater for 2 animal drinkers to reduce meter costs.