Left panel: Magnetic energy density in the ISM (red). We measured the energy density in an expanding sphere with a radius equal to the virial radius at the time. When the magnetic energy density inside this sphere grew for the first time by a factor of 10² compared to the adiabatically compressed field right after the protocluster collapse, we fixed the radius at this redshift. Right panel: Magnetic energy density in the ICM (blue) compared to the magnetic energy density expected from expanding the ISM magnetic field purely adiabatically to the volume of the ICM (red dotted). This estimate even overshoots the total magnetic energy in the ICM. The lower measured ICM magnetic field strengths, compared to adiabatically expanded ISM fields, may result from depletion during star formation, retention in the ISM, or unaccounted mixing losses.