According to one representative embodiment, a fluid storage device for storing a first fluid in a second fluid storage container fillable with a second fluid includes a first fluid storage container, a first fluid inlet coupled to the first fluid storage container, and a first fluid outlet coupled to the first fluid storage container. The first fluid storage container is flexible. The first fluid inlet is communicable in fluid receiving communication with a first fluid source. The first fluid in the first fluid storage container is caused to drain from the first fluid outlet based on at least one of a first fluid level, a first fluid storage container pressure, a second fluid storage container pressure, the second fluid storage container filling with the second fluid, the first fluid outlet being opened for first fluid egress, and a first fluid outlet pressure.