Sundula-
Congratulations on your recent breakthroughs in "The Canyon!" Now that you're hooking some of the larger fish, consider planning your strategy for playing and landing them before they are hooked.
Modern 6x and 7x tippets have adequate strength to land fish in the 20"+ range, provided that the tippet is in pristine shape and you can protect against sudden surges. Check for abrasion constantly, especially when nymphing. Try to keep a loop of line between the reel and your "control" finger that you can let slide (under slight tension) should the fish surge after being hooked. During the fight, lower the rod angle (to reduce friction) on subsequent surges, but apply all the pressure you can when you have the advantage. (The best angles are usually low and slightly to the rear of the fish.) Some who habitually fish light tippets incorporate a short "bungee" between the leader butt and the line as an additional "surge protector."
Survey your situation before you cast to a big fish. Will it be possible to keep the fish from charging downstream? If not, can you follow, or better yet, stay ahead of it? What are the obstacles the fish can use to its advantage, and are you in a position to steer the fish away from them?
The strategy may not always work the way you plan, but it is always better to have a strategy. Try to find an answer in advance to the old question, "Now that you've got it, what are you going to do with it?"
As for your question about the "toughest trout water," different streams pose different difficulties. Even streams that appear to be very similar can present their challenges in very different ways. Two local creeks come to mind. Both are tiny crystalline spring creeks, but that is about where the similarity ends.
One is virtually unfished--at least, I have never seen any sign of another angler's activity--and the fish are quite naive about flies. Despite this, it is one of the most difficult streams I have ever fished from an approach and presentation standpoint. If I can sneak into casting range and deliver the fly without spooking the fish, I'm almost guaranteed to have a take. I rarely need more than one fly pattern on this water; but, I need to play the weather and water conditions carefully to have the best shot at success.
The other little creek is undoubtedly one of the most heavily pressured streams in the world. About the only way to increase the pressure on this stream would be to throw a roof over it and light it for night fishing! On most days, I have no doubt that the trout see far more artificial flies than natural ones. Approach and presentation are of minor consequence, and what really matters is an understanding of how the fish respond to heavy fishing pressure.
Curiously, many anglers feel that heavy catch-and-release pressure can make fish virtually uncatchable, yet I have never been skunked on the latter stream in more than forty years of fishing it. On the former little creek, however, I run the risk of a skunking every time I fish it! Sometimes I work incredibly hard just to catch the smallest and most dimwitted of its trout. Each stream presents unique problems and each offers different rewards for solving those problems.