Qualified Sensors From Taiwan RiKO 

FRJ-9310-20 is a contact type fiber to detect the liquid level in a tank. It is covered with fluorine resin and can be used in a chemical agent. FRT-9310-20 has no metal parts on its body, so it can avoid releasing the metal ion into the liquid to reduce the quality of interface or to add its corrosiveness. It is with plastic parts only.

Theory of detection: The refractive index between the air and the fiber is big when the fiber tip is in the air. The emitted beam can be reflected back to the receiver from the boundary of fiber tip. On the contrary, the refractive index between the liquid and the fiber is small when the fiber tip is in the liquid. The emitted beam will scatter from the fiber into the liquid and can't be reflected back to the receiver. We can use this characteristic to detect if there is any liquid in a tank.

Custom design is welcome.