Martin,
The one environmental condition that has the biggest effect on our 'catching' of fish is the water temperature.
Tilapia react to cooler temps, perhaps more than native fish do. As the water cools, they become less agressive, but they will still feed.
In my experience, when it gets into the fall, Tilapia will strike very gently...often hard to tell they took the fly before they have spit it out again. They want a very slow, if not motionless presentation ,and they won't be as likely to 'chase' a fly. Depth control is critical.
Best way to combat all that is with a strike indicator. Light tippet with smaller flies, fished under an indicator, will let you 'see' subtle strikes, as well as give you absolute control over your depth and presentation speed.
Which flies you use is less critical, but somehthing like a small scud or anelid will work...so should a GRHE or Prince..or any generic small nymph pattern. They don't seem to be all that fly picky.
Good Luck!
Buddy
It Just Doesn't Matter....