I dont quite understand why you think that is so difficult 🙂 ..
How big size tank are they in ?
And why do you need to take them out now, why not wait untill they are a bit older and easier to catch.
I am really quite sure you can see the difference between the quindecim fry and any new fry hatching now – they will be much smaller.
I have often different size fry in my tanks, – its quite easy to see when there are ‘new babies’ around.
But of course – eventually you need to catch them out before they mature, – and if its a bigger tank than 20 liter, well, – then it is always a hazzle. But its possible 😉 .. you just need to empty the whole tank, though.