A fluid heating system includes a tank assembly having an interior surface bounding a chamber, the tank assembly having: a sidewall encircling the chamber and extending between a first end and an opposing second end, the first end bounding an opening to the chamber; and a lid movable between a first position wherein the opening to the chamber is exposed and a second position wherein the lid is disposed over the opening. A collapsible bag is removably disposed within the chamber of the tank assembly, the collapsible bag bounding a compartment adapted to hold a fluid. Systems are provided for controlling the temperature of fluid within the collapsible bag when the collapsible bag is positioned within chamber of the tank assembly. A mixing element is disposed within the compartment of the collapsible bag.