Abstract: A pre-manufactured fiberglass reinforced plastic (“FRP”) tank may be fitted with one or more conical internal diffuser plates (also termed nozzle plate, filtration plate, or chamber separator plate). To insert the diffuser plate into the FRP tank, a plastic liner is cut perpendicular to the axis of the tank and along the tank sidewall so as to allow complete access to the inside of the tank. The cut results in two opposing and substantially equal surfaces that mate with weld grooves on the diffuser plate. The first opposing face is welded to the mating weld groove on one side of the diffuser plate while the other opposing face is welded to the mating weld groove on the other side of the diffuser plate. The weld creates a hermetic seal between the interior of the tank and the exterior along both welds. The FRP tank may be cut a second time, or more times, to install additional diffuser plates as required.