The exposed red sensor is an ORP sensor. This connecter predates the connector to the right which includes a PVC sleeve and an o-ring sealed retainer nut.
Remove existing fittings for old style sensors
Remove old compression fitting
New Sensor Hardware
Install the new PVC sleeve into existing 3/4" tee.
New Sensor
Slide the new sensor into the sleeve and turn it clock-wise until tight.
New Cables
Connect the new style cables to the new sensor (s). Connect the open ended tinned leads into the controller as required.