Our Dessaltor DUO 60 manual, English version, states :
"The HP connectors should be installed strictly in accordance with the instructions (Appendix A2). Apply a little loctite or nut seal to the two male and female cones before tightening"
BUT this is not mentioned in the French version which came with the boat.
We have had leaks also from the tapered thread connector that screws into the membrane housing.
I use hydraulic thread sealant on all the metal to metal connections.