I use only Berkley Trilene 100% Fluorocarbon Professional grade, 2 Lb. The diameter is .006 This was the smallest diameter I could find and though is a bit pricey, find it to furl perfectly. I use it for fishing with my Intermediate line which is Rio Lake line. This Fluorocarbon sinks at the same rate as the line. I do use a 36 to 40 inch Fluorocarbon tippet. I have been fishing this set up on my high Mtn. lakes from my toon, using nymphs & emergers for 3 years now. I just cast out, count down and when I start getting strikes I know the depth. Works for me.