If the tank is made from molded styrene based FRG or even epoxy, it will most probably not be suitable to FDA.NSF regs/recommendations for POTABLE WATER. The 'emissions' or 'leachables' will most probably be way beyond the 'danger' zone if you can reconcile or prove a low-emission polyester or epoxy was originally used.

You can 'line' the tank internals with a NSF listed low amine "lining" epoxy .... but that will only bring the emissions 'close' to the regs. Better to start over with a polyethylene tank or a fabricated 316 stainless steel tank.