Fishing for large trout on flat water that are selectively feeding on small mayflies presents one of the most difficult challenges for fly fishers, especially so with highly pressured fish which is often the norm in many of our fisheries. If you want to become more successful when the fishing is super technical and demanding, quit using regular monofilament and switch to fluorocarbon tippet.
Fluoro is less visible in the water which increases the odds that a crafty fish will not see your leader, and it is more abrasion resistant which will increase your odds of landing a large trout on a light tippet. In addition mono absorbs water which weakens it, whereas fluoro does not absorb water and retains its strength which is also significant when hooking and playing large trout on light tippets. Some fly fishers may not even consider using fluoro for dry flies because it sinks. Fluorocarbon does sink, but in sizes 5x and smaller it does not break the surface film, and when presenting to rising fish your flies should land just a short distance above the fish so the time your fly needs to float is brief.
The majority of my technical fishing with small flies is during a Baetis hatch or a Trico spinner fall but I use fluorocarbon for all technical situations. I always fish two flies and start off with 6x or 7x fluoro to both flies, depending on how demanding the conditions are. For a Baetis hatch the highest percentage set up is a dun with a drowned emerger dropper. Highly pressured fish may become “dun shy” whereas they may take an emerger without hesitation.
My confidence combo is a Vis-A-Dun with a Barr’s Emerger dropper. For a Trico spinner fall I use a black bodied Vis-A-Dun with a sunk Trico spinner dropper. The black bodied Vis-A-Dun can be taken as a floating spinner, and the sunk spinner is often taken by even the most challenging trout. Some people are reluctant to use fluorocarbon tippet because of environmental concern that it takes too long to decompose. The reality is even though fluoro does take longer to break down than regular monofilament both take many years to deteriorate. Fluoro may take a thousand or more years to break down but regular mono still takes many hundred. Not sure where those stats come from but when researching the subject that is what I came up with.