strain
淥 centers on the action of straining or filtering a liquid, typically through a cloth or mesh, to separate solids.
淥 combines 氵 (water) with 彔, which likely contributes the sound and may relate to the idea of filtering or drawing off liquid. The exact historical development is uncertain.
Water 氵 flows through a filter-like shape 彔, leaving behind the solids. Picture pouring cloudy water through a cloth and watching it come out clear.
For ロク, imagine straining a thick liquid through a lock of wool: lock -> ロク, and the wool catches the particles as the water passes.