Bought these sensor to monitor the level through Apex of my custom DIY containers for two part. Ended selling the Apex DOS dosing pumps and never installed these. They are new. With these DIY cable, you plug it into your DOS and install the 2 optical sensors on 2 reservoirs of your choice at the warning level of your choice. There is a thread here on reef2reef about how to make these work and make the configuration. $40.00 shipped.