Two most commonly used standards for the design, construction, maintenance and inspection of water storage tanks, used for fire protection, are National Fire Protection Association (NFPA) standard 22 and and Factory Mutual (FM) standard 4020.
While both NFPA 22 and FM 4020 provide nearly similar provisions for the design of water storage tanks used for fire protection purposes, requirements of FM 4020 are of a higher level (more stringent).
By 'Seismic zone FM50', your sub-contractor would most probably be referring to the FM seismic zone with a 50-year mean recurring interval for the damaging ground motion.
For more details, you may refer to the following link:
http://www.pittsburgtank.com/wp-content/uploads/2015/05/FPCmag-FMvs.NFPA_.pdf
Regards.