I would like to know what flies are deadly for low, clear water conditions on rivers? And in hot weather like the weather what we are having at the moment.

And would a size 20 pheasant tail nymph catch fish? Should they be weighted or not? I have tied 2 with a pheasant thorax and 2 with a pearly thorax. But they are all non-weighted. Should I have weighted them?

Do fish actually go for flies as small as a size 20?

...Arctic...

Yes they certainly do, hooking & sticking with them is the big problem, raising them is not.
Trout unlike people do not have a "big boy" attitude to flies, they will feed on the most miniscule of fly given the chance. All we as anglers can do is hope to imitate them.
I think & I am at fault with the rest that we fish "to big" on many occasions.
We can learn a lot from our American friends about "fishing small". They regularly fish as small as a 22-26. I even seen a picture of a 32 once. Think Hans had something to do with it. Totally impractical in a general fishing scenario but it can be done. Tippets would have to fine down somewhat to accommodate the eye if nothing else. I am a great advocate of "fine & far off"
if low water condition persist.
Tight Lines

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"The Future's Bright The Future's Wet Fly"

In lower, clearer water fish become more fussy. Smaller flies are just more acceptable. Fish will take larger flies but generally speaking it will be due to instinct reactions - in faster water fish will snatch at flies whereas in slower gliding water they have time to inspect flies. Lets just say, smaller imitations do not spark a sense of danger as much as larger flies do.
I hope i've been of help,
Kind regards
Alex

Your little PTNs should be fine. Personally, I would add a little weight. A size 18 olive type nymph tied with a 1.5/2mm copper bead a the head works wonders for me in fast riffly water at this time of year. The weight gets it down that few inches that can sometimes make the difference.
If you fish it duo style beneath a reasonably large dry fly, it will really work. I think the big dry gets the trout's attention and even if they have no intention of taking it, often take the little nymph as it drifts past straight after.

Matt

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Across the fields of hoar. And the river steamed -
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In my experience anything below a size 16 will work. Keep it simple, a PTN,GRHE,rat or a lightly dressed spider. Nothing flashy and i does not have to be weighted or have a gold head, just degrease your leader and keep it light. Also dont bother going near the water untill about 9pm.
Hope this helps

If you come to the Derbyshire Wye, in the current heat wave, through the day you have two broad methods to try. Fish the streamy runs with a fake terrestrial, (cow dung, soldier beetle, hover fly, black fly) or skulk around under the trees in shaded pockets with a bushy suggestion such as a Red Hackle or a Double Badger. In the fast water you will find wild rainbow trout, under the trees you will find biggish browns...

richard