P.S. to my last post...

I would think that if all you are doing is screwing in a check-cap type fitting that you may not need any seal material: The fitting should be snug enough "as is" to maintain your siphon pressure.

The seal could even leak a small amount and still work assuming the siphon pressure is high enough. (Is the siphon outlet much lower than the water level inside your pond--The bigger the drop, thi higher the pressure, and the greater the siphon flow rate.

Good luck.