Fluidtrol FTP-56-8518 4" Vertical Modulating Dual Float Valve for Aquatic Systems. Our modulating type float valves are intended for use in aquatic systems where self-regulating flow is required. They work to make gradual adjustments to main drain flows based on surge tank water levels. Our horizontal and vertical float valves come standard with two butyrate float balls and feature polypropylene valve construction with stainless brackets and hardware.