I did not use any additional sealing in my setup.
So far, I had three incidents:
- after some time, when tanks were full, water pressure has dislocated lower right elbow and water started to leave the system forcibly. I have heard the noise, put elbow back in place and tied the junction with hay string to keep it better in place
- very heavy inflow of stormwater has disconnected overflow pipe from its socket. I have placed a heavy object on top of that junction to keep it in place
- I have stumbled upon water outlet valve, removing it from its socket, I have put it back in place.
One of the reasons for doing it all this way is that I want to be able to disassemble this system easily if a need arises (cleaning, broken pipe/tank, etc.) therefore I do not use any sealant and glues. If you want to make it more sturdy and secure, use silicone glue for aquariums perhaps.